Global climate forcing on late Miocene establishment of the Pampean aeolian system in South America

Domain: earth_science
Published: 2023-10-30
Editorial outcome: accept

Abstract
Wind-blown dust from southern South America links the terrestrial, marine, atmospheric, and biological components of Earth’s climate system. The Pampas of central Argentina (~33°–39°S) contain a Miocene to Holocene aeolian record that spans an important interval of global cooling. Upper Miocene sediment provenance based on n=3299 detrital-zircon U-Pb ages is consistent with the provenance of Pleistocene–Holocene deposits, indicating the Pampas are the site of a long-lived fluvial-aeolian system that has been operating since the late Miocene. Here, we show the establishment of aeolian sedimentation in the Pampas coincided with late Miocene cooling. These findings, combined with those from the Chinese Loess Plateau (~33°–39°N) underscore: (1) the role of fluvial transport in the development and maintenance of temporally persistent mid-latitude loess provinces; and (2) a global-climate forcing mechanism behind the establishment of large mid-latitude loess provinces during the late Miocene.

Introduction
The Pampas of central Argentina, South America (Fig. 1) record the production, transport, and deposition of dust in a region where wind-blown detritus has been shown to influence several components of the Earth’s climate system1–5. Lithogenic dust originating from South America alters radiative forcing budgets1,6 and provides micronutrients essential to photosynthetic organisms in the ocean and terrestrial sites that are capable of sequestering atmospheric CO2 7–10. In particular instances, aeolian detrital material serves as an important component of an internal feedback system: once initiated, arid, wind-dominated transport systems can be self-perpetuating, creating conditions that reinforce aridification and yield greater effluxes of aeolian dust11,12. Considering the potential ramifications for regional and global climate, reconstructing the long-term history of southern South American dust dynamics is critically important for understanding Earth’s climate evolution from the relatively warm climate of the mid-Miocene to the glacial-interglacial cycles of the Quaternary.

The Pleistocene–Holocene history of aeolian transport and deposition on the Argentine Pampas is recorded by ~700,000 km2 of aeolian sand and loess between ~30°–39°S latitude (Fig. 1)13. Miocene surface uplift of the Andes set the conditions for sediment supply to the lowlands and altered basin hydrology14,15. Most of the sediments exposed at the surface today accumulated during the late Pleistocene16, when large volumes of sediment from glaciated Andean watersheds were conveyed to the foreland by regional river systems17,18, which coupled with arid conditions in the Pampas, created ideal conditions for aeolian transport by westerly and southwesterly winds19–22.

Upper Miocene strata in the southwestern Pampas record pre-Quaternary aeolian transport and deposition. Globally, the late Miocene is associated with declining atmospheric pCO2 23, expansion of C4 plants24,25, declining sea surface temperatures, and increasing meridional sea surface temperature gradients26. Increased aridity and seasonality in the Andes during the late Miocene is linked to increased sediment production and deposition of aeolian strata27–29.

In the southwestern Pampas, the late Miocene climate is recorded by loess and loessic paleosols in the upper Miocene Cerro Azul Formation30. U-Pb detrital zircon data from the Cerro Azul Formation demonstrate that the fluvial-aeolian system responsible for upper Pleistocene–Holocene aeolian deposits on the Pampas was established and operating during the late Miocene, millions of years before the onset of latest Pliocene-Pleistocene glaciations. The fluvial-aeolian framework that has directed dust production, transport, and deposition in the Pampas consists of multiple components including sediment sources, foreland rivers and climate/atmospheric conditions conducive to aeolian transport. We propose this system has been in place since the late Miocene, responding to climatic drivers and orographic changes in the Andes, as well as drainage reorganization in the Andean foreland—a mechanism similar to that of the mid-latitude Chinese Loess Plateau in the Northern Hemisphere31–33.

The Pampas of central Argentina are a vast, low-relief expanse within the Andean foreland basin that extend from the foothills of the Andes in the west to the South Atlantic Ocean in the east, and the Sierras Pampeanas in the north and the Colorado Basin in the south (Fig. 1)34. Surficial deposits on the Pampas comprise the most extensive aeolian depositional system in South America, including an aeolian sand sea and a surrounding loess belt (~500 km long, 200 km wide) along its northern and eastern margins13,17,35,36. The majority of Pleistocene–Holocene aeolian deposits are vegetation-stabilized under current (interglacial) conditions37. Provenance data indicate these aeolian sediments were derived primarily from floodplain deflation of the Desaguadero, Colorado, and Negro rivers with additional input of volcaniclastic material from the Andean volcanic arc17,35,38–40. The present-day drainage configuration in this portion of the Andean foreland was generated as rock uplift migrated southward in the late Miocene to Pliocene, starting in the Sierras Pampeanas (26°–34° S) and moving south to include the Pampa Central block (35°–38°S) by the latest Miocene15,29,41. This resulted in topographic partitioning of the foreland. This partitioning caused the diversion of east-draining rivers southward, if present (e.g., ref. 39), and routed sediment to the Colorado Basin and the Atlantic coast between the Pampas and northern Patagonia39,42. The provenance of aeolian sediments deflated from megafans, bajadas (i.e., coalesced alluvial fans), and floodplains within the broken foreland has evolved in some areas in response to ongoing tectonics and changing catchment patterns39,43.

Along the southern and western margins of the Pampas, the Pleistocene–Holocene aeolian deposits are underlain by the upper Miocene Cerro Azul Formation (Fig. 1)30,41. The Cerro Azul Formation was deposited between ca. 8.9 and ca. 5.5 Ma based on the presence of Chasicoan and Huayquerian faunas across outcrops that delineate the late Miocene portion of the South America Land Mammal Ages/Stage framework30,41,44. The deposits consist of sandstone and siltstone; fluvial sandstones and paleosol horizons in the lower half of the formation are replaced by loess and paleosol deposits with pedogenic carbonate nodules and rhizoliths in the upper half30,45,46. The distribution of facies and stratigraphic thicknesses of the Cerro Azul Formation suggest deposition in the back-bulge depozone of the Andean retroarc foreland basin prior to uplift of the Pampa Central block, which segmented the foreland between ~35°S and ~38°S during the latest Miocene41. Aeolian deposits in the Cerro Azul Formation, recorded as loess-paleosol sequences, represent the oldest aeolian deposits in the Pampean Neogene succession and suggest generally more arid conditions in the late Miocene relative to the middle Miocene45. Greater aridity and/or seasonality during the late Miocene at 35°–38°S is consistent with rapid ecological and hydrologic changes across central South America during late Miocene cooling that followed the Middle Miocene Climatic Optimum47,48.

Results
We collected N=9 samples of loess and loessic paleosol from exposures of the upper Cerro Azul Formation across a ~300 km east-west distance (Fig. 1) and analyzed these using U-Pb detrital zircon geochronology. Results are presented in Fig. 2 as histograms and kernel density estimate (KDE) plots, which display a smoothed distribution of detrital zircon ages from the Cerro Azul Formation. Sample locations, individual sample KDE plots, tabulated records, isotopic ratios, elemental concentration data, and uncertainties are included in the Supplementary Materials (Fig. S1 and Tables S1, S2). Maximum depositional ages were calculated using the maximum likelihood age algorithm49 and yield ages generally between ca. 9 and 6 Ma (Table 1). Although these ages represent the maximum possible depositional age and not necessarily the actual depositional age, they are consistent with existing age constraints for the upper half of the Cerro Azul Formation30,44.

All samples contain eight U-Pb zircon age populations, or modes, that are shared between samples and across locations (Fig. 2 and S1). Most of the zircon grains likely represent some degree of recycling; however, their U-Pb ages are consistent with sediment source rocks in the region. The eight age modes are: [1] 5–25 Ma, derived from the Andean magmatic arc50; [2] 50–125 Ma, derived from the Andean magmatic arc, including the North Patagonian batholith, or more specifically, megafans sourced from this area51; [3] 150–200 Ma, derived from the Chon Aike Silicic Large Igneous Province52; [4] 220–280 Ma, derived from the Choiyoi Magmatic Province53; [5] 330–400 Ma, derived from rocks associated with a Devonian-Carboniferous magmatic arc exposed in the San Rafael and Chadileuvú blocks54; [6] 420–475 Ma, derived from plutonic rocks associated with the Famatinian arc55; [7] 480–540 Ma, derived from igneous rocks of the Pampean Orogeny and early magmatic activity of the Famatinian Orogeny56; and [8] 900–1200 Ma, derived from rocks of the Cuyania terrane, exposed west of the present-day Desaguadero basin56.

The relative abundance of each population varies between samples, although the 220–280 Ma population is the largest age mode for all nine samples, with the 330–400 Ma mode being the second largest for all samples. When combined into a single KDE plot (Fig. 2), the relative abundance of each of the detrital zircon age modes are, from largest to smallest: [1] 220–280 Ma; [2] 330–400 Ma; [3] 900–1200 Ma; [4] 420–475 Ma; [5] 480–540 Ma; [6] 50–125 Ma; [7] 150–200 Ma; and [8] 5–25 Ma.

Methods
Sampling and mineral separation
We collected N=9 samples of loess and loessic paleosol from exposures of the upper half of the Cerro Azul Formation located along the southwestern margin of the Pampas (Fig. 1). Three samples were collected from the type section of the Cerro Azul Formation near the Rio Desaguadero30; the others were collected from outcrops across a ~300 km east-west transect (Fig. 1). Measured sections were completed at sampled outcrops. Samples were processed for detrital-zircon recovery using density and magnetic mineral separations. Detrital zircon grains with sizes of silt to very-fine sand were extracted from samples using standard separating procedures76. All disaggregated sample material was passed through a 500-µm sieve and <500 µm material was further separated using a Gemeni water shaker table. Grains were separated using a Frantz barrier field isodynamic magnetic separator and differentiated using lithium metatungstate heavy liquid. Zircon grains were mounted in 2.5 cm diameter epoxy and polished with 2000 grit sandpaper to expose grain cores.

Laser-ablation inductively-coupled-plasma mass-spectrometry
U-Pb ages of individual grains were measured using laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) at the Arizona LaserChron Center of the University of Arizona, yielding a total of n=3299 U-Pb ages with a range of n=198–541 per sample, allowing some robust comparison between the relative proportions of ages within age modes77. Laser spot size used for analyses was 20 µm. Elemental and mass-fractionation instrument drift and down-pit fractionation were corrected using a suite of reference materials mounted with the zircon samples. The reference material included Sri Lanka (‘SL’), FC-1, and R3376. The corrections were made using an in-house, Microsoft Excel-based software (AgeCalc). The 206Pb/238U versus 207Pb/206Pb age cut-off used was 900 Ma thus zircon crystal ages with 206Pb/238U ages <900 Ma were calculated using the 206Pb/238U ratio and crystal ages with 206Pb/238U ages >900 Ma calculated using the 207Pb/206Pb ratio. We employed the following rejection criteria: [1] maximum 204Pb signal intensity of 100 cps; [2] maximum 206Pb/238U uncertainty of 10%; [3] maximum 207Pb/206Pb uncertainty of 10%; [4] maximum 206Pb/238U vs. 207Pb/206Pb age discordance of 30%; or [5] maximum 207Pb/206Pb vs. 206Pb/238U reverse age discordance of 10%.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this paper, Stubbins et al. document a potentially long (since Miocene) record of an aeolian-fluvial\nsystem in South America. These types of long records are very rare, and so this is quite noteworthy.\nBased on their convincing age and provenance data, aeolian activity continued (perhaps not\nuninterrupted) from the Miocene into the Pleistocene-Holocene. This is based on their data showing\nthat the provenance of loess from Miocene and Pleistocene deposits has remained unchanged aside\nfrom more recent volcanic contributions to the Pleistocene loess. Rivers draining the Andes are a key\nsource of sediment for this long-lived aeolian system and connects the importance of the tectonic\ndevelopment of the Andes to this aeolian system. The story is straightforward and significant by\nhighlighting the importance of dust in the Southern Hemisphere and linking tectonics, fluvial and aeolian\nsystems, and climate change, increasing its relevance to a broad audience. The importance of aeolian-\nfluvial systems is being recognized more frequently in recent research, and this will be an important\ncontribution to that work.\nThe paper makes an interesting comparison to the Chinese Loess Plateau, suggesting the two are linked\nby global climate change and Hadley cell circulation patterns. I find this link exciting and compelling.\nThe writing is very good and the logic is easy to follow, aside from a few long sentences. I found the\nfigures generally easy to read and interpret (I had to study Fig 1 to see where the Cerro Azul Fm was\nlocated, as the dotted pattern doesn’t come through very well in the key), and the supplemental figures\nwere useful to show the strong similarities in provenance between the Miocene and Pleistocene-\nHolocene loess. I recommend publication with minor edits.\n62: Did aridity and seasonality directly increase the sediment production and deposition? The sentence\nskips some steps, so to soften that, I suggest the following phrasing: aridity and seasonality is linked to\nincreased sediment production and deposition.\n163-168. Long sentence. I suggest breaking this up into two or revising.\n190-194: Another long sentence. If kept intact, replace allowing with allowed. And perhaps replace the\ndashes with commas.", "reviewer_2": "Summary:\nThis is a very interesting paper that uses detrital zircons to explore an important question about the rise\nof continental-scale aeolian systems. Their main finding is that pooled DZ spectra from the Miocene\nCerro Azul Formation match well with the pooled DZ spectra from the aeolian sands and loss on the\nmodern Argentine Pampas. They then build on this similarity to propose that the aeolian system was\ninitiated in the Miocene and has persisted until today. Ultimately they extend their findings to argue for\na global forcing mechanism to establish large mid latitude loess provinces.\nAlthough these conclusions are interesting and plausible, the potential impact of the ‘big ideas’ is\ndiminished by their relatively unconvincing arguments for the persistence of the Pampean aeolian\nsystem. For example, the core finding of the paper is that the pooled DZ spectra are similar between the\nMiocene Cerro Azul Fm. and the modern Pampean sand and loess. They note that the pooled spectra\nare quite different from other regional sources such as the Puna and Patagonia. To me, this seems like a\nvery intuitive finding given that the sand and loess is found down wind of the Pampean river systems\nthat generate it. The key conclusions I would draw from this dataset are that the pooled DZ spectra are\nconsistent with the geology of the Pampean source region and have been blown to the east, likely\nrecycled many times as the climate has oscillated between drier and wetter conditions. It is not clear\nwhether the aeolian system has truly persisted since the Late Miocene, or whether zircon crystals\nthemselves have persisted as they were intermittently pushed eastward. To me this is an important\ndistinction that can only be resolved by more careful stratigraphic synthesis, demonstrating that there\nare actually eolian deposits that span the entire time interval between Miocene and present.\nI suggest that the authors could do more to demonstrate that the source regions are truly identical\nbetween the Miocene Cerro Azul and Pleistocene samples. I worry that pooling the DZ spectra masks\nsome complexity that could point to a more complex story. I propose an additional figure showing the\nindividual spectra as CDF (or KDE). It would also be a good idea to include all of the DZ analyses in a\nsupplemental data table for use by future researchers.\nIn sum, my main comment is that the paper should be reframed a bit and presented in a more nuanced\nway to directly address some of the major assumptions that underly the mote speculative conclusions.\nSpecific comments:\nLines 54-55: My intuition is that relatively little of the Andean foreland sediment is late Pleistocene in\nage. That said, please clarify your thinking. Are you talking about foreland basin sediments or the sand\nveneer covering the more distal Pampas?\nLines 60-62: This comment makes it sounds like the late Miocene cool conditions continued directly into\nthe latest Pliocene/Pleistocene. Don’t forget about the Pliocene warm period, which likely marked a\nreturn to warm temps between the late Miocene and Pleistocene. I suggest bringing in a bit more\ndetailed background info about what is known about the Pliocene terrestrial climate of Argentina here.\nAdmittedly, this may not be much…\nLines 68-70: To me it is intuitive that this system has been in place in some form since the rise of the\nAndes. Rivers bring sediment to the foreland and it is blown west. I think the paper would benefit from a\nmore nuanced analysis of this question that includes a broader stratigraphic synthesis. Are there aeolian\ndeposits older than the Cerro Azul Fm.? Are their aeolian deposits during the Pliocene to signal the\nsystem persisted? I recognize that later in the paper the authors discuss the relative lack of Pliocene\ndeposits, suggesting they have been largely deflated.\nLines 81-84: This seems like the key point to me.\nLines 90-92: Is this an alternative ‘straw hypothesis’. Consider developing it further and clarifying it. If\nthis straw hypothesis is correct, would it predict something very different from your findings?\nLines 109-112: I feel it is important to present the DZ spectra from all samples individually, without\npooling them. At least as a supplementary figure. Although the MDS technique is exciting, to me it is not\na replacement for presentation of the full DZ spectra. This would allow the results to be visualized\nwithout the overlay of a fairly complex and opaque statistical technique. Individual spectra could be\nplotted either as KDE or CDF. The CDF might be more appropriate as it directly relates to the KS statistic\nthat underpins the MDS technique.\nLines 139-141: Can you offer more detail on what method you are using to estimate this similarity\ncoefficient? Also, can you give us a sense of how similar the samples within each of the pooled groups\nare (i.e. within the Cerro Azul and Pleistocene groups).\nLines 159 to 160: It could be helpful to explain how the other regional samples were obtained and\nestimated. Are they from pooled river samples? Pooled bedrock samples? Is there potential for grain\nsize biasing between these depositional environments and an aeolian environment?\nLines 173-174: Here I think it is important to clarify that you show evidence for ‘nearly identical’ aeolian\nzircon spectra. Not sure it is justified to jump to the conclusion that the systems were nearly identical.\nSeems that would require a more details stratigraphic analysis.\nLines 181-183: Similar comments to above. It is not clear to me that the Pleistocene samples were\ntransported from their original bedrock source during rh Pleistocene, but instead are likely reworked\nover many climate cycles.\nLines 204-206: This is an important point to strengthen your arguments. Can it be further developed?\nAre their Pliocene aeolian deposits elsewhere in the study area?\nLines 211-213: Clarify what you mean by ‘delimited’ here. Are you essentially saying that the overlying\nPliocene aeolian sediments were eroded down to the duricrust, which prevent deeper erosion?\nLines 213-220: Great that you are addressing what might have happened during the Pliocene. Seems this\nuncertainty undermines the strength of the argument for a single identical system persisting from\nMiocene to Pleistocene.\nLines 266-269: What grain size was ultimately analyzed? Was further sieving performed prior to\nmounting/analysis? Is there any concern over grain sized biasing when comparing loess-derived zircons\nto fluvial systems, etc?\nFigure 1: Can you clarify what is meant by loess samples? Those are only Cerro Azul samples, correct?\nFigure 2: Would still like to see individual KDEs or CDFs for each sample. Consider labeling y-axis as “#\nof grains” or similar.\nFigure 3: At some point it would be great to unpack how these other comparison spectra for SP, Chaco,\nPatagonia, and Puna were compiled and what geologic context they are from\nFigure 4: might label ‘westerlies’ in red text to match arrows.", "reviewer_3": "In this study, Stubbins et al. present new U-Pb ages of detrital zircons, largely of late Miocene age, from\nthe Argentinian Pampas region. Their data show that the provenance of these grains is very similar to\nthose found in Pleistocene and Holocene sediments and hence that the fluvial-aeolian processes\nresponsible for the Pampas system have been in operation since the late Miocene. They argue that this\nresult demonstrates that global cooling was responsible for the generation of large mid-latitude loess\nprovinces in the late Miocene.\nThe conclusion that there has been little change in the provenance of sediments within the Pampas\nsince the late Miocene is generally well supported by the new data presented here with a clear and\ntransparent methodology. However, I find that “global climate forcing on the late Miocene\nestablishment of the Pampean system” (as stated in the title) is not currently sufficiently evidenced\nwithin the manuscript to support this as the main conclusion of this study. Most of the evidence for this\nsecond conclusion seems to come from previous work, and very little of this is shown within this paper\n(although relevant studies are cited and their conclusions stated in the text). In addition, several of the\ncited references already make the link between late Miocene aridification and regional sedimentation\n(e.g. 27-29), so I think a clearer statement of how this study represents a significant advance on what is\nalready known and/or tackles an outstanding question would be beneficial.\nIn order to prove that global climate forcing was responsible for the establishment of the Pampean\nsystem, I think there are four key points that need to be clearly demonstrated: 1) The Pampas was\nestablished in the late Miocene. 2) A dominant local tectonic driver of this event can be ruled out. 3)\nThere was local climatic change at this time that can explain the establishment of the Pampean system.\n4) This climatic change can be linked to/explained by global factors. Regarding each of these four points:\n1) There are some new ages presented in this paper, which are reported to be consistent with previous\nwork. If these new ages are adding additional value, this could be more clearly emphasised to help\nstrengthen this manuscript. Also, as the timing of loess formation is key to the main conclusions, a figure\nor panel demonstrating the distribution of reported ages would be helpful. The late Miocene spans ca. 6\nmillion years. Can you be any more precise about the likely timing of Pampas establishment and/or\ncomment on whether it was rapid or gradational?\n2) The text currently states that the “fluvial-aeolian system is the product of Andean tectonics, which\nprovides a steady influx of sediment and influence lower-level patterns, combined with more arid\nconditions that emerged during the late Miocene”. Can you comment on how important these two\ndrivers are relative to each other? This is crucial to how strongly the message that global climate was the\nmain driver can be stated but is not currently discussed in detail.\n3) L190-196 list a range of previously published evidence for regional climate change. This is really\nimportant to the key message, and I think the paper would benefit from plotting some of these data (or\na summary of their results) in a figure to allow readers to see this evidence for themselves. This could be\ncombined with the age data in point 1.\n4) Links are made in the text to both late Miocene cooling and the development of the Chinese Loess\nPlateau in the text, but again, no evidence is directly presented to the reader within this study to\ndemonstrate how similar timings/patterns of change are to justify the strength with which this key\nmessage is presented. This could also be shown as part of a figure.\nIn summary, I find that this study presents new provenance data that clearly demonstrate the relatively\nstability over millions of years of a major fluvial-aeolian system, and has the potential to be a paper\nworthy of publication in Nat. Comms. However, in my opinion, the main conclusion of the paper is not\ncurrently well enough supported to justify publication in the current form. I therefore suggest that\neither the authors provide more evidence to support their conclusions so that their key message is well\nsupported without requiring further reading (my recommended option to produce a manuscript of\ninterest to a broad audience), or they change the manuscript title and reduce the confidence with which\nthey link their records to climatic drivers.\nLine-by-line comments\nL31: “a global forcing mechanism” is quite vague.\nL35-36: This implies that there was no wind-blown dust before the establishment of the Pampas, which\nseems unrealistic!\nL44: Which instances?\nL44-50: There’s a bit of a reasoning jump here. Why is S. America in particular “critically important”?\nL58, L64: “western Pampas” v “southwestern Pampas” – is the distinction significant? As a general point,\nplease make figure 1 tie into the text more closely so that a reader unfamiliar with the study area can\neasily identify the features described.\nL64-65: Reference?\nL64-L72: It’s not clear what’s being stated as already known and what’s presented as the novel findings\nof this study in this paragraph.\nL71: “similar mechanism” rather than “thesis”.\nL90-92: Phrase this as a hypothesis? Are you trying to set up the idea that large province shifts might still\nbe expected in the latest Miocene and that’s something that your data will allow you to explore?\nL91: Define bajadas\nL93-106: Some repetition here with L58-72.\nL103-L104: More arid/greater aridity and/or seasonality than what?\nL108-134: More explanation would be useful throughout the results section to help a non-specialist\naudience.\nL109-110: A comment on the geographical spread of the samples would be useful here.\nL110-111: What is/is the purpose of a KDE?\nL116: What about the lithology of lower half of the Cerro Azul formation? Is this relevant in the\naridification history of the region?\nL117: Explain “U-Pb zircon age modes”.\nL138-139: From the same locations? Or averaged across region/exposures?\nL143: “the same” rather than “equivalent”?\nL145: “all samples” stated earlier – do some Pleistocene samples differ?\nL159-162: The phrasing of this sentence is quite confusing\nL163-168: What determines which sources are the most important? Wind/river direction? Erodibility?\nExtent of exposure? Other?\nL173-178: This is an example of a section where clearer integration with Fig 1 would be helpful.\nL183-185: How important are these two factors? References needed for late Miocene aridification.\nL187: Unclear which part of the sentence “possibly” refers to.\nL185-189: How do deposition rates change?\nL204-206: Confusing – “may have” and “it did so” seem to contradict each other. Are you saying it\npersisted in one location but not extensively?\nL218: Any other lines of evidence to support this change in Hadley cell dynamics?\nL237: Three of your 9 samples have MLA >9Ma, so can the formation of the Pampas be directly linked to\nthe 8Ma start of the CLP? This age discrepancy should be acknowledged and discussed.\nL244: Why is the similarity in spatial footprints relevant?\nL249: “would be needed” instead of “had to occur”.\nL255-257: “the global nature of our findings” currently not supported by the presented data\nL267: Clarify if >500 or <500μm fraction kept.\nL292-293: Part of sentence missing?\nL314: “Simplified” not “general” model to avoid any confusion with GCMs?\nL324: Please carefully check reference formatting e.g. L350\nL686: Figshare link not currently working, but I assume would be made live upon acceptance?\nFigure 1: I find this figure messy and overly complex. I suggest aligning it more closely with the aspects\nof local geography discussed in the text to help the reader follow the main arguments. Is it possible for\nthe geological units shown to more closely align with the 8 age modes used in the study? What does all\nof the white space represent? Is the Cerro Azul represented by dots on both white and yellow\nbackgrounds? Figure would look smarter if framed by a box. Also ensure latitude tick marks are outside\nedge of map (40S appears to be within national boundary).\nFigure 2: Add y-axes labels & ensure tick marks touch axis.\nFigure 3: Label axes. Need to acknowledge source of background image in panel B?\nFigure 4: Add “aeolian production areas” to legend. Can’t see lossoid deposits anywhere on the map?\nWhat is the significance of the different arrow colours? Define SALLJ.\nS1: Ensure CaCO3 formatted correctly throughout. What is “Type 1 Locality”?"}, "author_response": "Authors Responses to Reviewers’ comments\n(AR = Authors’ Response; line #s refer to “clean” manuscript, sans “track changes”)\nReviewer 1:\n62: Did aridity and seasonality directly increase the sediment production and deposition? The sentence\nskips some steps, so to soften that, I suggest the following phrasing: aridity and seasonality is linked to\nincreased sediment production and deposition.\nAR: we have rephrased, as suggested. Line 65-66\n163-168. Long sentence. I suggest breaking this up into two or revising.\nAR: we have broken this into two sentences. Line 175\n190-194: Another long sentence. If kept intact, replace allowing with allowed. And perhaps replace the\ndashes with commas.\nAR: we have changed the sentence following the reviewer’s advice. Line 218\nReviewer 2:\nGeneral comments:\na) It is not clear whether the aeolian system has truly persisted since the Late Miocene, or whether zircon\ncrystals themselves have persisted as they were intermittently pushed eastward. To me this is an\nimportant distinction that can only be resolved by more careful stratigraphic synthesis, demonstrating that\nthere are actually eolian deposits that span the entire time interval between Miocene and present.\nAR: The reviewer raises an interesting point that requires a bit of clarification.\nThe reviewer is asking the hypothetical, or alternative, hypothesis: What if the sediment\nwas initially delivered to the foreland basin during the Miocene and simply been\nrecycled over the last ~6 million years?\nAs the reviewer mentions, this is an important consideration. We agree that some\nsediments may have been recycled over time, particularly amongst the sand-sized\nfraction. However, our samples consist of loess and loessoid deposits, and it is highly\nunlikely loess would be continuously recycled within the same basin over the past ~6\nmillion years. There are 4 considerations that do not support the ‘recycling’ hypothesis:\n1) There is not enough material in the Cerro Azul Formation to account for the\nvast Pleistocene-Holocene deposits (and Pliocene deposits as well). This necessitates\na continued influx of sediment from the Andes to the foreland since the Late Miocene.\nWe added a statement to the text to clarify this: Line 195-197\n2) The ~2 m calcrete horizon that overlies the Cerro Azul Formation serves as an\n“erosional cap,” preventing extensive erosion of the Cerro Azul Formation. This means\nthat overlying eolian sediment in the basin must have been derived from coeval foreland\nriver systems and not recycled. This is discussed in Line 238-251\n3) Although not stated explicitly, implicit within Line 245, where the low\npreservation potential of loess is discussed, lies the notion that recycling the same\nloess-sized material (silt) within the same basin is highly unlikely.\n4) Pliocene eolian deposits exist in central Argentina east of our study area,\nsupporting a relatively continuous aeolian system between Miocene-Holocene – in\ncontrast to a recycled hypothesis. Lines 239-240.\nTwo other issues are tangentially raised in this discussion, which we have\naddressed. When discussing this fluvial-aeolian system, we are not only describing the\naeolian system, but also the requisite components of the system, which include the\nsediment source (various ranges within the Andes), basin, foreland rivers, and\natmospheric-climate conditions conducive to eolian transport. The near-identical detrital\nzircon record from late Miocene eolian deposits and Holocene eolian deposits makes it\nclear that this ‘system’ was in place during the late Miocene and has been operating\nsince that time (i.e., persisted). To clarify this, we have made various changes (see\nspecific details below) to better define this in the text, including: “…The fluvial-aeolian\nframework that has directed dust production, transport, and deposition in the Pampas\nconsists of several components including sediment sources, foreland rivers and\nclimate/atmospheric conditions…” Line 72-73\nWith respect to the general comments - A (Part 1):\nWe agree with the reviewer that this does not necessarily mean that the aeolian\nsystem was active throughout that entire duration – certainly climate cycles and other\nvariables have impacted aeolian activity in the Pampean aeolian system over time. This\nis mentioned several times in the text as well – e.g., Line 246-255\nWith respect to the general comments B:\nb) I suggest that the authors could do more to demonstrate that the source regions are truly identical\nbetween the Miocene Cerro Azul and Pleistocene samples. I worry that pooling the DZ spectra masks\nsome complexity that could point to a more complex story. I propose an additional figure showing the\nindividual spectra as CDF (or KDE). It would also be a good idea to include all of the DZ analyses in a\nsupplemental data table for use by future researchers.\nThe Reviewer brings up a very good point regarding the data included in this\nmanuscript. The pooling of ages into Miocene and Pleistocene samples is done\nintentionally in order to make 1st order comparisons between the two aeolian systems\n(Miocene versus Pleistocene). In other words, the focus of this study is to make the\nbroader interpretations, which is most easily done by pooling ages. However, pooling\ndata can obfuscate the individual sample results to some extent. In the case of this,\nstudy, this is not a concern – the individual Miocene samples display little to no variation\nand are consistent with the pooled ages. Nonetheless, the reviewer is correct in the\nassertion that we need to have these individual samples available to the reader.\nTo rectify this, we have created a new figure (FigS1 – Supplementary Figure 1)\nof all of the individual KDEs from each sample and added this as an additional figure for\nthe supplementary data section.\nSpecific comments From Reviewer 2:\nLines 54-55: My intuition is that relatively little of the Andean foreland sediment is late Pleistocene in age.\nThat said, please clarify your thinking. Are you talking about foreland basin sediments or the sand\nveneer covering the more distal Pampas?\nAR: yes, we are referring to the material at the surface today. We’ve altered the text\naccordingly in order to make this clear. Line 57\nLines 60-62: This comment makes it sounds like the late Miocene cool conditions continued directly into\nthe latest Pliocene/Pleistocene. Don’t forget about the Pliocene warm period, which likely marked a\nreturn to warm temps between the late Miocene and Pleistocene. I suggest bringing in a bit more\ndetailed background info about what is known about the Pliocene terrestrial climate of Argentina here.\nAdmittedly, this may not be much…\nAR: Because this is the Introduction, we are hesitant to add details that could distract\nfrom the key points. However, we discuss this issue later in the text - Lines 235-237\nThat being said, paleoclimate records indicate an overall cooling since the late\nMiocene. It is true that the mid-Pliocene warm period (or mid-Piacenzian warm period)\nis associated with increased temperatures, but this is a relatively minor departure from\nthe overall trend. Moreover, temperatures during mid-Pliocene warming were still cooler\nthan those in the late Miocene (Prevosti et al., 2021; Herbert et al., 2016 and references\ntherein). As the reviewer suggests, information on the Pliocene terrestrial climate of\nArgentina is sparse, and the Pliocene sedimentary record in our study area is non-\nexistent.\nLines 68-70: To me it is intuitive that this system has been in place in some form since the rise of the\nAndes. Rivers bring sediment to the foreland and it is blown west. I think the paper would benefit from a\nmore nuanced analysis of this question that includes a broader stratigraphic synthesis. Are there aeolian\ndeposits older than the Cerro Azul Fm.? Are their aeolian deposits during the Pliocene to signal the\nsystem persisted? I recognize that later in the paper the authors discuss the relative lack of Pliocene\ndeposits, suggesting they have been largely deflated.\nAR: We made changes to the Background section – adding that “Aeolian deposits in the\nCerro Azul Formation, recorded as loess-paleosol sequences, represent the oldest\naeolian deposits in the Pampean Neogene succession …” Line 109-110. We did not\nmake additional changes to this specific part of the manuscript (in the Introduction)\nbecause we are simply trying to introduce the overall arc of the study.\nLines 81-84: This seems like the key point to me.\nAR: No change requested.\nLines 90-92: Is this an alternative ‘straw hypothesis’. Consider developing it further and clarifying it. If this\nstraw hypothesis is correct, would it predict something very different from your findings?\nAR: Both Reviewer #2 and #3 had a question about this sentence.\nWe have redrafted it to make it a clear statement – a statement that summarizes our\ncurrent understanding of how provenance has changed slightly in response to recent\ntectonic uplift. This has been demonstrated in Garzanti et al., 2022. The primary point of\nthe sentence (beyond the explicit statement) is to make the reader aware that this is a\ntectonically active foreland and therefore sediment provenance is constantly evolving as\ncrustal blocks are displaced and eroded. Line 96-98\nLines 109-112: I feel it is important to present the DZ spectra from all samples individually, without\npooling them. At least as a supplementary figure. Although the MDS technique is exciting, to me it is not a\nreplacement for presentation of the full DZ spectra. This would allow the results to be visualized without\nthe overlay of a fairly complex and opaque statistical technique. Individual spectra could be plotted either\nas KDE or CDF. The CDF might be more appropriate as it directly relates to the KS statistic that\nunderpins the MDS technique.\nAR: We have added a figure to the supplementary section (Figure S1) as the reviewer\nrequests. We agree that the individual samples should be shown individually.\nLines 139-141: Can you offer more detail on what method you are using to estimate this similarity\ncoefficient? Also, can you give us a sense of how similar the samples within each of the pooled groups\nare (i.e. within the Cerro Azul and Pleistocene groups).\nAR: We have included the reference, which contains the details of how the similarity\ncoefficients were calculated (Saylor and Sundell 2016). Similarity within or between\nsamples is now accessible by observing our newly added figure (Figure S1).\nLines 159 to 160: It could be helpful to explain how the other regional samples were obtained and\nestimated. Are they from pooled river samples? Pooled bedrock samples? Is there potential for grain\nsize biasing between these depositional environments and an aeolian environment?\nAR: we amended the text a bit and added:\n“In Figure 3, samples from the Cerro Azul Formation are plotted alongside samples\ncollected from modern rivers, surficial sediment and bedrock samples.” Line 169\nLines 173-174: Here I think it is important to clarify that you show evidence for ‘nearly identical’ aeolian\nzircon spectra. Not sure it is justified to jump to the conclusion that the systems were nearly identical.\nSeems that would require a more details stratigraphic analysis.\nAR: we have changed the text slightly to address this point. We are not trying to say that\nthe Miocene eolian system is identical to what we see today – e.g., with large eolian\ndunes (Pampean Sand Sea) and a loess belt. We are trying to say that the source\nareas and sediment transport pathways are the same.\nWe added this point to the text to correct this. Line 186\nLines 181-183: Similar comments to above. It is not clear to me that the Pleistocene samples were\ntransported from their original bedrock source during rh Pleistocene, but instead are likely reworked over\nmany climate cycles.\nAR: This is a good point – and we discuss this at some length in the “general\ncomments” section above. We added that there is some possible recycling. However,\nthere is no way that the Cerro Azul Formation, even if all of it were recycled, could\naccount for the volume of Pliocene and Pleistocene eolian strata in central Argentina.\nThere has to have been a continuous supply of sediment and eolian entrainment and\ndeposition throughout the Miocene-Holocene (see response under general comments\nabove).\nWe added to the text: “Recycling of some sediment from the Cerro Azul Formation into\nPliocene and Pleistocene-Holocene deposits may have occurred, although this is\ninferred to have been limited owing to the erosion-resistant calcrete beds above the\nCerro Azul Formation (cite 43). The volume of Pliocene and Pleistocene-Holocene\nsedimentary deposits in the Pampas requires a prolonged influx of sediment to the\nforeland. Line 238-251\nLines 204-206: This is an important point to strengthen your arguments. Can it be further developed? Are\ntheir Pliocene aeolian deposits elsewhere in the study area?\nAR: There are no Pliocene units preserved in our study area. However, there are\naeolian units to the east along the Atlantic margin in a region known as the Colorado\nBasin. We highlighted this and added the location to the text:\n“of eastern Argentina (Fig. 1)” Line 261-262\nLines 211-213: Clarify what you mean by ‘delimited’ here. Are you essentially saying that the overlying\nPliocene aeolian sediments were eroded down to the duricrust, which prevent deeper erosion?\nAR: Yes, this is correct. There is a ~2 m thick pedogenic carbonate above the Cerro\nAzul Formation that acts as a “cap” to the unit – it is probably the main reason that the\nCerro Azul Formation has not been eroded away.\nWe added to the text:\n“The resistant calcrete bed above the Cerro Azul Formation prevented the erosion of\nthe upper Miocene strata.” Line 248-249\nLines 213-220: Great that you are addressing what might have happened during the Pliocene. Seems this\nuncertainty undermines the strength of the argument for a single identical system persisting from\nMiocene to Pleistocene.\nAR: Our discussion of the fluvial-aeolian system refers to the aeolian deposits, but also\nencompasses the source areas, the foreland rivers, the basin, the winds, sufficient\naridity/seasonality, orographic blocking, etc – everything that enables the aeolian\nsediments to be deposited. Our data indicates this was emplaced during the late\nMiocene. But directly to the review’s point, yes and no. With no appreciable relative\nuplift in the study area since the late Miocene (aside from a few meters of uplift of the La\nPampa Central block relative to the Chadileuvú block and Macachin Basin), the lack of\na Pliocene-early Pleistocene record across the study area is quite intriguing. As an\nunderfilled portion of the flexural foreland, one would think if alluvial-fluvial dominated\ndeposition reappeared during the Pliocene, there would be a record of it somewhere.\nHowever, with fine-grained aeolian deposits, the preservation potential in the geologic\nrecord is quite low, making preservation the exception, not the rule—that was the\nMeijer and van der Meulen (2023) reference in the original manuscript. Following the idea of\npreservation potential, either deposition of fine-grained aeolian strata coupled with\nrepeated intervals of erosion, or effectively no accumulation because of aridity, become\nthe most plausible explanations for the lack of Pliocene to early Pleistocene strata in the\nstudy area. We lean towards deposition with erosion, rather than zero accumulation\nbecause there is certainly no evidence for hyperaridity, but rather drier with more\nseasonality. There are always exceptions, but generally speaking, the accumulation of\nfine-grain aeolian deposits requires some vegetation (or significant surface roughness).\nOf course, we aimed to simplify this in the manuscript so as to not enter into the territory\nof wild speculation. Given what we have learned from studying wind-eroded bedrock\nfloored basins in China and the probability that the windward margin of the Chinese\nLoess Plateau has retreated due to wind erosion (e.g., (Kapp et al., 2015; Stevens et\nal., 2018), and applying it to the idea of the calcrete duricrust in the study area delimiting\nsubsequent (wind) erosion, we tried to present the reader with a credible scenario that\nis internally consistent with the (current understanding of the) regional geology...which is\nlimited because of the limited Pliocene-early Pleistocene record. Perhaps coupled flux\nand provenance investigations of dust in the South Atlantic record would shed light on\nthe Pliocene uncertainty.\nIf the reviewer can think of how we can add clarity for the reader regarding how the\nPliocene-early Pleistocene fits into the model, we are open to suggestions.\nLines 266-269: What grain size was ultimately analyzed? Was further sieving performed prior to\nmounting/analysis? Is there any concern over grain sized biasing when comparing loess-derived zircons\nto fluvial systems, etc?\nAR: We have added to the text the grain size we analyzed – “grains with sizes of silt\nand very fine sand” Line 310.\nThe reviewer raises an interesting point in that there can always be some bias based on\ngrain sizes. However, our conclusions are based on the comparison between generally\nthe same-sized grains in upper Miocene strata and Pleistocene-Holocene units.\nTherefore, therefore concerns are dampened. There is the possibility of some\ndiscrepancies between fluvial sediments and the loess we sampled; however, this\nwould not change our primary conclusion in any way (same provenance, same\npathways, same fluvial-aeolian system). Additionally, the upper Miocene and\nPleistocene-Holocene samples were prepared and analyzed in a self-similar fashion,\nthus minimizing laboratory-induced biases which could to erroneous interpretations\nduring sample comparisons.\nFigure 1: Can you clarify what is meant by loess samples? Those are only Cerro Azul samples, correct?\nAR: Correct. To clarify this we added a brief line to the figure caption: Line 336-337.\nFigure 2: Would still like to see individual KDEs or CDFs for each sample. Consider labeling y-axis as “#\nof grains” or similar.\nAR: Added the individual KDEs – Figure S1. We altered figure 2, adding Y-axis labels.\nFigure 3: At some point it would be great to unpack how these other comparison spectra for SP, Chaco,\nPatagonia, and Puna were compiled and what geologic context they are from\nAR: We agree. However, we are using these data to represent general source areas;\nthere is a risk of overinterpretation at this point in our understanding. We feel the\nconservative and most prudent use of these data is to use them as general sediment\nsource areas so as to better understand the Pampean aeolian sediments. Further\ncompilations and data sources will allow for additional interpretations.\nFigure 4: might label ‘westerlies’ in red text to match arrows.\nAR: changed Figure 4 (now Figure 5).\nReviewer #3\nGeneral Comments I:\nThe conclusion that there has been little change in the provenance of sediments within the Pampas since\nthe late Miocene is generally well supported by the new data presented here with a clear and transparent\nmethodology. However, I find that “global climate forcing on the late Miocene\nestablishment of the Pampean system” (as stated in the title) is not currently sufficiently evidenced\nwithin the manuscript to support this as the main conclusion of this study. Most of the evidence for this\nsecond conclusion seems to come from previous work, and very little of this is shown within this paper\n(although relevant studies are cited and their conclusions stated in the text). In addition, several of the\ncited references already make the link between late Miocene aridification and regional sedimentation\n(e.g. 27-29), so I think a clearer statement of how this study represents a significant advance on what is\nalready known and/or tackles an outstanding question would be beneficial.\nRegarding this general comment and comment #III below, we have added a\nfigure (now Figure 4) which shows a synthesis of surface uplift in Asia and South\nAmerica, the expansion of C ecosystems in Asia and South America, a δ18O benthic\n4\nstack, and sea surface temperatures (SSTs). This figure provides a visual reference for\nthe reader to go along with the references which were included in the original\nmanuscript. Additional references were added to the manuscript through this process. In\nsummary, this figure shows that orographic blocking of westerly sourced moisture to the\nPampas was emplaced well before the deposition of the Cerro Azul Fm.\nAdditionally,\nthis figure shows the expansion of C ecosystems in Asia and South America overlaps\n4\nthe depositional age of the upper (i.e., aeolian) Cerro Azul Fm. Finally, and most\nimportantly for our interpretations, the figure shows a pronounced increase in meridional\nSST gradients between the tropics and mid-latitudes in both hemispheres during the\nlate Miocene when the Cerro Azul Fm. was deposited. This increase in the meridional\nSST gradients largely resulted from the cooling of the mid-latitudes SSTs. This change\noccurred in both hemispheres, and because of the robust link between Hadley\nCirculation and low-to-mid-latitude SST gradients across timescales (e.g., (Bjerknes,\n1966; Brierley et al., 2009; Feng et al., 2018; Hong et al., 2023; Li et al., 2023; Lindzen,\n1994; Seo et al., 2014; Terray et al., 2003) and many others), this provides quite\ncompelling corroborating evidence for our hypothesis that a shift in the characteristics\n(i.e., intensity and/or latitudinal extent) of the Hadley cells drove changes in mid-latitude\naridity and dust dynamics during the late Miocene.\nThe focus of this study is on the aridification of the Pampas during the late\nMiocene. Assuming elevations in the Andes have increased or remained steady over\nthe past 10 million years, which is a safe assumption given all we know of Andean\ntectonics, there are no viable tectonic processes that could have led to the aridification\nof central Argentina. Moisture in the Pampas was derived from eastern (South Atlantic\nOcean) and northern sources, including the South American Low Level Jet (SALLJ),\nwhich transports moisture from the tropics. Whereas the SALLJ is created by the barrier\nimposed by the Central Andes, increasing surface elevations within the Andes would\nhave only served to increase the SALLJ and thus moisture delivery to central Argentina\n(Carrapa et al., 2018). Similarly, a rise in elevation in the Argentine Andes would have\nlikely led to increased precipitation (e.g., Insel et al., 2010), which is counter to the\nobserved aridification. Orographic effects would not impact the Pampas because the\nregion does not receive significant precipitation from westerly winds, nor is there any\nevidence they did so during the late Miocene. There is no question that the Andes play\na role in local and regional climate, but there are no viable, documented changes to the\nAndes during the late Miocene that could explain more arid conditions in the Pampas.\nIn contrast, changes in Hadley Cell circulation would have been highly effective\nin causing more arid conditions in the Pampas and surrounding regions during the late\nMiocene. The so-called late Miocene cooling of SSTs is well documented (Herbert et\nal., 2016) by a host of paleoclimate indicators. Paleoclimate models of South America\nindicate this change in late Miocene temperatures would have resulted in contraction, or\nintensification, of Hadley Cell circulation. The models indicate the intensified Hadley\nCells would have reduced moisture delivery to central Argentina (Carrapa et al., 2018).\nThere is plenty of evidence that the region became more arid during the late Miocene,\nwhich include the onset of aeolian sedimentation in the Pampas as well as those\nreferences cited in the text.\nTo support the new figure additional text has been added to the Discussion:\nLines 207-228\nRegarding the contribution of this work, we argue that the data, interpretations\nand discussions provided in this manuscript represent a significant advance in our\nknowledge of South American dust dynamics, wind regimes, and climate more broadly.\nThis assertion is supported by comments from Reviewers 1 and 2. Firstly, we\ndemonstrate the onset and continuity of the Pampean eolian system, one of the largest\ncontinental aeolian systems in the world. Knowledge of when this system began and\nhow it has evolved did not exist prior to these data. Secondly, we show that the onset of\nthis system coincided with regional aridification that can be tied to changes in synoptic\ncirculation during the late Miocene, which again was unknown prior to this study. And\nthirdly, we tie the onset of the Pampean aeolian system in the southern hemisphere to\nthe development of the Chinese loess plateau in the northern hemisphere. To our\nknowledge, this is the first attempt to provide a bi-hemispheric hypothesis that brings\ntogether the origins of two large continental loess/aeolian provinces. This hypothesis will\nspring a number of follow-up studies including those in sedimentation, climate modeling,\nand sediment-climate feedback systems.\nGeneral Comments II:\nIn order to prove that global climate forcing was responsible for the establishment of the Pampean\nsystem, I think there are four key points that need to be clearly demonstrated: 1) The Pampas was\nestablished in the late Miocene. 2) A dominant local tectonic driver of this event can be ruled out. 3)\nThere was local climatic change at this time that can explain the establishment of the Pampean system.\n4) This climatic change can be linked to/explained by global factors.\nRegarding the 4 proofs requested by the reviewer:\n1) “The Pampas was established in the late Miocene”\nWe altered the text to show that the Late Miocene Cerro Azul Formation contains the\nfirst evidence of aeolian strata in the Pampas (Lines 109-110) – hence the\nestablishment of the Pampean aeolian system. We also added that the lower portion of\nthe Cerro Azul Formation is devoid of aeolian deposits.\n2) “A dominant local tectonic driver of this event can be ruled out”\nWe added statements to discuss why the tectonic hypothesis is unviable (Lines 207-\n228). There is no scenario where tectonics in the Andes can produce more arid\nconditions in the Pampas – orographic barriers of westerly winds were not (and are not)\nrelevant to precipitation in the Pampas. If anything, surface uplift in the Andes would\nhave led to greater precipitation by focusing the SALLJ towards the Pampas (ceteris\nparibus). In contrast, GCM models demonstrate that changes to synoptic-scale climate\npatterns (e.g., Hadley Cells) are effective in shutting off moisture to the Pampas.\n3) “There was local climatic change at this time that can explain the establishment of the Pampean\nsystem”\nLines 217-220 contains a list of data that indicate local climate change “evidenced in\ncentral and southern South America through carbon and oxygen isotope data in\npedogenic carbonates and fossil enamel, and fossil crown height” - Carrapa et al., 2018,\nHynek et al., 2012, and Bywaters Reyes et al., 2010. In addition, the appearance of\neolian strata in the Cerro Azul Formation provides a line of evidence that local climate\nhas changed.\n4) “This climatic change can be linked to/explained by global factors”\nWe added that climate models indicate global cooling of sea surface temperatures\nduring the Late Miocene lead to climate change in South America – “Climate models\nindicate more arid conditions in the late Miocene Pampas and Sierras Pampeanas\nresulted from changes to synoptic-scale atmospheric circulation patterns that reduced\nmoisture export from tropical South America”. (Lines 207-215)\nGeneral Comments III:\nFour points raised by the reviewer:\n1) There are some new ages presented in this paper, which are reported to be consistent with previous\nwork. If these new ages are adding additional value, this could be more clearly emphasised to help\nstrengthen this manuscript. Also, as the timing of loess formation is key to the main conclusions, a figure\nor panel demonstrating the distribution of reported ages would be helpful. The late Miocene spans ca. 6\nmillion years. Can you be any more precise about the likely timing of Pampas establishment and/or\ncomment on whether it was rapid or gradational?\n2) The text currently states that the “fluvial-aeolian system is the product of Andean tectonics, which\nprovides a steady influx of sediment and influence lower-level patterns, combined with more arid\nconditions that emerged during the late Miocene”. Can you comment on how important these two\ndrivers are relative to each other? This is crucial to how strongly the message that global climate was the\nmain driver can be stated but is not currently discussed in detail.\n3) L190-196 list a range of previously published evidence for regional climate change. This is really\nimportant to the key message, and I think the paper would benefit from plotting some of these data (or\na summary of their results) in a figure to allow readers to see this evidence for themselves. This could be\ncombined with the age data in point 1.\n4) Links are made in the text to both late Miocene cooling and the development of the Chinese Loess\nPlateau in the text, but again, no evidence is directly presented to the reader within this study to\ndemonstrate how similar timings/patterns of change are to justify the strength with which this key\nmessage is presented. This could also be shown as part of a figure.\nRegarding the 4 questions/topics raised by the reviewer:\n1) Late Miocene age? Can we be more specific?\nNew figure added (now Figure 4 – climate and tectonics synthesis).\nAt this point there is simply not enough data to be more specific about the age of these\nunits. However, this uncertainty mirrors uncertainties around the expansion of C4\necosystems in both hemispheres. The most up-to-date data from this region is\npresented in Prevositi et al., 2021 and is incorporated into our results. Whereas more\nrefined chronostratigraphy would be very welcomed, it is not a part of this study, nor\nwould it alter the main conclusions. Speculating the precise ages of the beds is\ntempting, but not scientific. We have included our maximum depositional age\ncalculations (MDAs) to provide greater confidence that these beds were deposited\nduring the late Miocene. However, MDAs do not reflect actual depositional ages – they\nonly provide a constraint by giving the oldest age a bed could possibly be. We added\nsome text to clarify this – Line 123-127\n2) Relative role of tectonics?\nLate Miocene moisture in the Pampas was provided by eastern sources (Atlantic\nOcean) and northern sources (South American Low-Level Jet) – which is still the\ndominant pattern today. Tectonics (which we assume means surface uplift in the Andes)\nwould have little impact on the 1st order characteristics of this system. Some uplift could\ncause more precipitation in the Andes near the Pampas, but overall, Andean tectonics\ndid not play a large role in this area. No moisture is associated with westerly winds in\nthese latitudes so there would not have been any orographic effect.\nIn summary, tectonics do not appear to have played the primary role. The relative role is\nstill unknown, but is probably best investigated with climate model studies – which are\nnot in the purview of this manuscript.\nTo address this reviewer’s comment, we tied to make this more explicit in the text -\n(Lines 207-228 and new Figure 4).\n3) Climate data\nWe attempted to add these data to Figure 1, but we subsequently removed it because it\nsimply added more material to an already complicated image. Instead, we added a brief\nsentence to the figure caption (1) stating that the paleoclimate data we reference in the\ntext were collected from the Sierras Pampeanas. – Line 341-342\n4) Link to Loess Plateau\nIn an attempt to not relitigate the timing of the establishment of the Chinese Loess\nPlateau, we provided four key references (Refs. 65–69 [original]) to the reader.\nWe\ncould have selected references from an exhaustive list all showing approximately the\nsame thing (i.e., there was a massive expansion in the loessic deposits on the Ordos\nPlateau around 7–8 Ma). Why the range one might ask. 1) This is, in part, because the\nbasal loessic sediments were filling in existing topography which had significant relief.\nTo that end, the initial deposition was non-uniform. 2) Like the Cerro Azul Fm.,\npaleontology and magnetostratigraphy are the primary means of depositional age\ncontrol. Both approaches lack the precision needed to claim the onset of deposition with\nmuch precision.\nLine-by-line comments\nL31: “a global forcing mechanism” is quite vague.\nAR: we added “global-climate” forcing mechanism. Line 34\nL35-36: This implies that there was no wind-blown dust before the establishment of the Pampas, which\nseems unrealistic!\nAR: Our understanding is that this a teaser, meant to provide a brief overview of the\nmanuscript topic, and not a synopsis of the aeolian history of South America.\nFurthermore, we do not believe this teaser implies there was never any wind-blown dust\nprior to this time. However, if this is an issue we can propose an alternative teaser.\nL44: Which instances?\nAR: We have provided two references that can be read to explore such instances. Line\nAR: In our view, the logic is clear – South America provides large amounts of wind-\nblown detritus (and the macronutrients and micronutrients it carries) to the South\nAtlantic, which could ultimately help sequester CO2. It may be part of a larger positive-\nfeedback cycle (more dust = colder climate = more glaciers = more dust…). Therefore, it\nis critical to understand the long-term history of dust and eolian activity in this region.\nL58, L64: “western Pampas” v “southwestern Pampas” – is the distinction significant? As a general point,\nplease make figure 1 tie into the text more closely so that a reader unfamiliar with the study area can\neasily identify the features described.\nAR: Good point. We changed this and following instances to make this more consistent.\nE.g., Line 62\nL64-65: Reference?\nAR: we have added a reference for this. Line 69\nL64-L72: It’s not clear what’s being stated as already known and what’s presented as the novel findings\nof this study in this paragraph.\nAR: As none of this material is cited, it understood to be new information from this\nstudy, which will be outlined in the following text – – with the exception of the\ninformation related to the Chinese Loess Plateau, which has citations.\nL71: “similar mechanism” rather than “thesis”.\nAR: changed Line 77\nL90-92: Phrase this as a hypothesis? Are you trying to set up the idea that large province shifts might still\nbe expected in the latest Miocene and that’s something that your data will allow you to explore?\nAR: Both Reviewer #2 and #3 had a question about this sentence – so we changed it to\nmake it more evident that it is a statement rather than a subtle question. It is a\nstatement that summarizes our current understanding of how provenance has changed\nslightly in response to recent tectonic uplift. This has been demonstrated in Garzanti et\nal., 2022. The primary point of the sentence (beyond the explicit statement) is to make\nthe reader aware that this is a tectonically active foreland and therefore sediment\nprovenance is constantly evolving as crustal blocks are displaced and eroded. Line 96-\nAR: The term “Bajada” – coalesced alluvial fans – is a common term in geology and\nphysical geography. However, recognizing that this a multidisciplinary journal the text\nnow includes “(i.e., coalesced alluvial fans)”. Line 96-97\nL93-106: Some repetition here with L58-72.\nAR: The former lines (93-106 on the original submission) are a brief introduction to the\nstrata and the area as part of the introduction. These latter lines are part of the\n“Background” and include a more detailed description.\nL103-L104: More arid/greater aridity and/or seasonality than what?\nAR: Good point. We altered the text to say ‘…middle Miocene’. Line 110-111\nL108-134: More explanation would be useful throughout the results section to help a non-specialist\naudience.\nAR: In general, we have tried to do this – primarily though the changes requested in the\nfollowing comments.\nL109-110: A comment on the geographical spread of the samples would be useful here.\nAR: We added that this is across a 300 km distance Line 118\nL110-111: What is/is the purpose of a KDE?\nAR: This is more common in the sediment provenance world, but it is basically a\nsmoothed histogram. We include both the histogram and the KDE in the figures. We\nchanged the text to provide some information on the KDE - Line 120-122\nL116: What about the lithology of lower half of the Cerro Azul formation? Is this relevant in the\naridification history of the region?\nAR: Good point. We added some information to the “Background” stating that the lower\nhalf of the formation consists of fluvial sandstones and paleosol horizons (rivers and\nfloodplains) and loess and paleosols in the upper half. Line 103-106. overall, this is\nconsistent with aridification during the late Miocene.\nL117: Explain “U-Pb zircon age modes”.\nAR: Corrected – we have added “Age populations, or modes…” to help explain this. A\nmode or population is the term given to groups of detrital zircons with the same ages.\nLine 128\nL138-139: From the same locations? Or averaged across region/exposures?\nAR: Added that the data are combined into one KDE. Line 149 We also added “Figure\nS1” to the list of figures – this figure shows that the individual KDEs are identical to the\nPooled KDE. This concern was also raised by Reviewer 2.\nL143: “the same” rather than “equivalent”?\nAR: Yes. Changed this. There is no reason to avoid the word “same” Line 154\nL145: “all samples” stated earlier – do some Pleistocene samples differ?\nAR: Changed this to “the samples” – there is also Figure S1 that corroborates this. Line\nAR: agreed. Changed this to what we think is easier to comprehend: “MDS plots show\nthe greatest similarity between the Cerro Azul Formation U-Pb data and the Colorado,\nNegro, and Desaguadero Rivers, which are known to be important nodes in the dust\nproduction pathway…” Line 171-175\nL163-168: What determines which sources are the most important? Wind/river direction? Erodibility?\nExtent of exposure? Other?\nAR: This is a tricky question and one we as a community are still trying to tease apart. A\nnumber of variables play a role – amount of zircons in the source rock (i.e., fertility),\nerodibility, size of the source area and sediment delivery system, etc. – all of which vary\nbetween locations and through time. In short, the large number of uncontrolled variables\nprecludes any reasonable answer to this question, and we are hesitant to speculate in\nerror. However, we do suggest one likely cause for the similarity between the Rios\nColorado and Negro and the dissimilarity with the Rio Deseaguadero. There is\ngeological evidence to support the idea that the Colorado and Negro rivers existed in\nsome form during the late Miocene (i.e., they were supplying sediment), whereas most\nof the evidence suggests the Deseaguadero River did not exist at that time. Therefore it\nis not surprising that the Cerro Azul Formation has more affinity with the sediment in the\nColorado and Negro rivers. Line 175-178\nL173-178: This is an example of a section where clearer integration with Fig 1 would be helpful.\nAR: This is fair. Figure 1 shows all of the features mentioned in this section, although to\nsomeone unfamiliar with the area, it may take some time to absorb the details. We\nadded a reference to Figure 1 in the text Line 190, Line 224. In addition, we added a bit\nmore explanation of the aeolian system to the figure caption of Figure 1 – Line 339-342.\nWe hope that these two changes will make it easier to follow the text.\nL183-185: How important are these two factors? References needed for late Miocene aridification.\nAR: This is another good question, but one that cannot be quantitatively addressed at\nthis time, and certainly not with this data set. Both are needed – a sediment source and\nconditions conducive to eolian transport, thus we did not attempt to discuss which may\nhave been more or less important. Added the citation requested – Line 202\nL187: Unclear which part of the sentence “possibly” refers to.\nAR: We removed ‘possibly’ Line 203-206\nL185-189: How do deposition rates change?\nAR: Interesting question. We do not address this for two reasons: 1) without adequate\nchronological constraints, sedimentation rate estimates are of very limited to no use.\nThese units lack the age constraints needed to reconstruct an effective sedimentation\nrate. 2) This study examines a ~300 x 100 km area. The sedimentation rate is very\nsensitive to the location selected in the Pampas and central Argentina. In some areas,\nthere are no Pliocene strata preserved, but in other parts (e.g., Colorado Basin) there\nare relatively thick Pliocene deposits. The location would greatly impact rates.\nL204-206: Confusing – “may have” and “it did so” seem to contradict each other. Are you saying it\npersisted in one location but not extensively?\nAR: Corrected – removed “may have” – Line 238-244\nL218: Any other lines of evidence to support this change in Hadley cell dynamics?\nAR: At this point, not that we are aware of. In this part of the discussion we are trying to\npresent potential new avenues of research.\nL237: Three of your 9 samples have MLA >9Ma, so can the formation of the Pampas be directly linked to\nthe 8Ma start of the CLP? This age discrepancy should be acknowledged and discussed.\nAR: Maximum depositional ages are relatively imprecise tools that indicate a particular\nbed has to be younger than X Ma (you cannot have a 10 myr old bed with 8 myr old\nzircons). They are not necessarily coincident with depositional ages. We added a brief\nexplanation of this to the Results section - “Although these ages represent the\nmaximum possible depositional age, and not necessarily the actual depositional age,\nthey are consistent with existing age constraints” – Line 123-127\nL244: Why is the similarity in spatial footprints relevant?\nAR: In this section we are attempting to show the similarities between these two\ncontinental aeolian systems. An important component of that is to demonstrate they are\nof the same size.\nL249: “would be needed” instead of “had to occur”.\nAR: changed to “would have needed” Line 290-291\nL255-257: “the global nature of our findings” currently not supported by the presented data\nAR: We removed the “global” part of this and altered it to say that the parallels between\nthe Chinese Loess Plateau and the Pampas suggests late Miocene climate changes\ninfluenced mid-latitude conditions and deposition. – Line 298-301\nL267: Clarify if >500 or <500μm fraction kept.\nAR: changed this to show that we kept the <500 µm material. There are virtually no\nzircons >500 µm. Further processing and separation of such large grains is therefore\npointless. For our samples, this was not a problem because most of our grains were\n<100 µm. This is a standard procedure, but we have changed the text to clarify this. –\nLine 312\nL292-293: Part of sentence missing?\nAR: This is not an issue in our draft, but we will certainly fix this if it remains a problem.\nL314: “Simplified” not “general” model to avoid any confusion with GCMs?\nAR: Excellent point. Text has been changed. Line 377\nL324: Please carefully check reference formatting e.g. L350\nAR: We have gone back and checked formatting.\nL686: Figshare link not currently working, but I assume would be made live upon acceptance?\nAR: Correct. An embargo will remain in place on the Figshare folder until this\nmanuscript has finished the peer review process and accepted. All data were provided\nwith this submission.\nFigure 1: I find this figure messy and overly complex. I suggest aligning it more closely with the aspects of\nlocal geography discussed in the text to help the reader follow the main arguments. Is it possible for the\ngeological units shown to more closely align with the 8 age modes used in the study? What does all of the\nwhite space represent? Is the Cerro Azul represented by dots on both white and yellow\nbackgrounds? Figure would look smarter if framed by a box. Also ensure latitude tick marks are outside\nedge of map (40S appears to be within national boundary).\nAR: This is a fair point. We spent a significant amount of time altering Figure 1 so that it\nis easier to comprehend, but also maintains all of the necessary information.\nWe have simplified the geologic map and muted the colours, which enhances the\nQuaternary and Miocene aeolian strata as well as the sample locations. We are hesitant\nto break up the geology to correspond with the age modes because the exposures of\nage mode rocks may not correspond to all sources – For example, Mesozoic and\nCenozoic sedimentary rocks contain a lot of 220-280 Ma zircons, in addition to the\nexposures of 220-280 rocks. To keep it simple and less biased, we simply show the\nmodern geological exposures.\nWe added a box around the figure, corrected line widths, and more palatable.\nIn general, we believe the revised figure 1 is more in line with the reviewer’s request – it\nlessens the geologic map and enhances the key features that are discussed in the text.\nTo the figure caption we’ve added that the paleoclimate data discussed in the text is\nfrom the Sierras Pampeanas.\nFigure 2: Add y-axes labels & ensure tick marks touch axis.\nAR: corrected figure 2\nFigure 3: Label axes. Need to acknowledge source of background image in panel B?\nAR: The axes in MDS plots are typically not labeled because they represent dissimilarity\nvalues that are dependent upon the dataset – they are not absolute values, but values\ngenerated to fit the data. We added the citation for the background figure in the figure\ncaption Line 364\nFigure 4: Add “aeolian production areas” to legend. Can’t see lossoid deposits anywhere on the map?\nWhat is the significance of the different arrow colours? Define SALLJ.\nAR: We added these items to Figure 4 - the legend and made the loessoid deposits\nmore prominent. We’ve added the “westerlies” label for the red arrows. Added the\nSALLJ definition to the figure caption Line 386.\nS1: Ensure CaCO3 formatted correctly throughout. What is “Type 1 Locality”?\nAR: These comments refer to the Table S1 in the supplementary files – we have\ncorrected the formatting and we added a statement to note that “Type 1 Locality” refers\nto the type section of the Cerro Azul Formation as defined in Visconti et al., 2010.\nBjerknes, J., 1966, A possible response of the atmospheric Hadley circulation to equatorial\nanomalies of ocean temperature: Tellus, v. 18, no. 4, p. 820-829.\nFeng, J., Li, J., Jin, F., Zhao, S., and Zhu, J., 2018, Relationship between the Hadley circulation\nand different tropical meridional SST structures during boreal summer: Journal of\nClimate, v. 31, no. 16, p. 6575-6590.\nHerbert, T. D., Lawrence, K. T., Tzanova, A., Peterson, L. C., Caballero-Gill, R., and Kelly, C.\nS., 2016, Late Miocene global cooling and the rise of modern ecosystems: Nature\nGeoscience, v. 9, no. 11, p. 843-847.\nHong, Q., Feng, J., and Li, J., 2023, Modulation of the Central Pacific El Niño on the\nRelationship between the Hadley Circulation and Tropical SST: Journal of Geophysical\nResearch: Atmospheres, p. e2022JD038195.\nKapp, P., Pullen, A., Pelletier, J. D., Russell, J., Goodman, P., and Cai, F., 2015, From dust to\ndust: Quaternary wind erosion of the Mu Us Desert and Loess Plateau, China: Geology,\nv. 43, no. 9, p. 835-838.\nLi, Y., Du, M., Feng, J., Xu, F., and Song, W., 2023, Relationships between the Hadley\ncirculation and tropical sea surface temperature with different meridional structures\nsimulated in CMIP6 models: Frontiers in Marine Science, v. 10, p. 1145509.\nLindzen, R. S., 1994, Climate dynamics and global change: Annual Review of Fluid Mechanics,\nv. 26, no. 1, p. 353-378.\nSeo, K. H., Frierson, D. M., and Son, J. H., 2014, A mechanism for future changes in Hadley\ncirculation strength in CMIP5 climate change simulations: Geophysical Research\nLetters, v. 41, no. 14, p. 5251-5258.\nStevens, T., Buylaert, J. P., Thiel, C., Újvári, G., Yi, S., Murray, A. S., Frechen, M., and Lu, H.,\n2018, Ice-volume-forced erosion of the Chinese Loess Plateau global Quaternary\nstratotype site: Nature Communications, v. 9, no. 1, p. 983.\nTerray, P., Delécluse, P., Labattu, S., and Terray, L., 2003, Sea surface temperature\nassociations with the late Indian summer monsoon: Climate Dynamics, v. 21, p. 593-\n618.\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nThanks for making the suggested changes.\nReviewer #3 (Remarks to the Author):\nI thank the authors for a thorough consideration of the feedback provided by the reviews to produce a\nclearer and better argued manuscript. I now recommend this study for publication in Nature\nCommunications.\nMinor points\nFig 1 caption: “Chadileuvú block (CB), Pampa Central block (PCB), and San Rafael block (SRB).” Seems to\nbe an incomplete sentence?\nFig 4 & caption: Westerhold et al. 2020 is a benthic isotope composite record, not a stack.\nLine-by-line response to reviewer comments\nReviewer #2 (Remarks to the Author):\nThanks for making the suggested changes. The authors thank the reviewer for the measured\nguidance and handling of the review process.\nReviewer #3 (Remarks to the Author):\nI thank the authors for a thorough consideration of the feedback provided by the reviews to\nproduce a clearer and better argued manuscript. I now recommend this study for publication in\nNature Communications. Agreed. We thank you for your contribution to improving this\nmanuscript.\nMinor points\nFig 1 caption: “Chadileuvú block (CB), Pampa Central block (PCB), and San Rafael block\n(SRB).” Seems to be an incomplete sentence? Agreed. Lines 341–342 now read:\n“Abbreviations are as follows, Chadileuvú block (CB), Pampa Central block (PCB), and San\nRafael block (SRB).”\nFig 4 & caption: Westerhold et al. 2020 is a benthic isotope composite record, not a stack. Good\ncatch. Replaced “stack” with “composite record”."}]

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