Reconciling Southern Ocean fronts equatorward migration with minor Antarctic ice volume change during Miocene cooling

Domain: earth_science
Published: 2023-11-09
Editorial outcome: minor_revision

Abstract
Gradual climate cooling and CO2 decline in the Miocene were recently shown not to be associated with major ice volume expansion, challenging a fundamental paradigm in the functioning of the Antarctic cryosphere. Here, we explore Miocene ice-ocean-climate interactions by presenting a multi-proxy reconstruction of subtropical front migration, bottom water temperature and global ice volume change, using dinoflagellate cyst biogeography, benthic foraminiferal clumped isotopes from offshore Tasmania. We report an equatorward frontal migration and strengthening, concurrent with surface and deep ocean cooling but absence of ice volume change in the mid–late-Miocene. To reconcile these counterintuitive findings, we argue based on new ice sheet modelling that the Antarctic ice sheet progressively lowered in height while expanding seawards, to maintain a stable volume. This can be achieved with rigorous intervention in model precipitation regimes on Antarctica and ice-induced ocean cooling and requires rethinking the interactions between ice, ocean and climate.

Introduction
Temperature contrasts between the equator and high latitudes are mitigated through poleward atmospheric and ocean heat transport1,2. Variability in the latitudinal sea surface temperature (SST) gradient is mostly a function of polar temperatures, which are much more variable than those at low latitudes because of polar amplification3. In turn, polar SSTs, especially offshore Antarctica, vary with prevailing cryosphere conditions, including sea ice extent4,5. The steepest part of the latitudinal SST gradient is at mid-latitudes, at the boundary between subtropical gyres and subpolar waters. In the Southern Hemisphere, this is the subtropical front (STF): the northern limit of the Southern Ocean and the Antarctic Circumpolar Current (ACC), and the centre of ocean carbon uptake6 (Fig. 1). The ACC and associated oceanographic fronts, driven by westerlies and steered by bathymetry7, regulated deep ocean ventilation8–10 and heat exchange between low and high latitudes11,12. In turn, the latitudinal position of westerlies is influenced by the extent of sea ice around Antarctica13,14. Oceanographic conditions around the ocean fronts thus play a central role in the latitudinal distribution of heat in the Southern Hemisphere, including the heat source that causes basal melt and instability of marine-terminating Antarctic ice sheets6. Future projections of polar climate change, and the consequences for cryosphere melt and sea level are highly uncertain15, because changes in and interactions between Antarctic ice sheets, sea ice and oceanography bear numerous poorly constrained, non-linear feedbacks6,16. Important constraints on the functioning of this system in a warming world might come from reconstructions of geologic episodes during which the partial pressure of atmospheric CO2 (pCO2) was as high as projected for the future.

Throughout the Neogene (23–2.58 Ma), pCO2 declined from 800 to 300 parts per million (ppm)17, global temperatures dropped18,19, latitudinal SST gradients increased20 and global ice volume19,21–23 and sea ice expanded24. The current paradigm assigns pCO2 decline as the primary driver, which, through polar amplification of cooling, stimulates ice growth and cooling in the regions of deep-water formation18,25,26. Yet, recent data have challenged this view. A recent study found that Neogene SST gradients increased in the subtropical gyre but decreased from the subtropical front to polar waters27. With relatively stable equatorial28,29 and polar SSTs30, this indicates that the mid-latitudes, rather than the high-latitudes27, cooled most profoundly in the Neogene. Antarctic-proximal records suggest a retreated Antarctic ice sheet and warm Antarctic-proximal conditions24,30,31 during the mid-Miocene Climatic Optimum (MCO) and profound seaward ice sheet advance during subsequent cooling termed the Middle Miocene Climatic Transition (MMCT)21,23,32–34, in line with pCO2 estimates35. Along with a rise in deep ocean benthic foraminifera oxygen isotope ratios (δ18Obf), this suggests a strong increase in global ice volume. Yet, the existing series of clumped isotope measurements (Δ47, which deconvolves temperature and ice volume components in δ18Obf records) on Miocene benthic foraminifera36–38 suggest higher-than-previously-estimated Bottom Water Temperatures (BWTs) during the MCO, and as a result, large global ice volume. These records also indicate strong BWT cooling during the MMCT, explaining most if not all of the δ18Obf rise, and therefore little to no ice volume build up. How this generally connects to far-field sea level changes is still poorly reconciled. However, the uncertainties in clumped isotope data and the limited resolution and temporal range of the records leave ambiguity on the true amount of BWT drop and ice volume build up during the mid-Miocene.

Like the modern, changes in the Southern Ocean, notably regarding fronts and currents, were likely vital for heat transport towards the ice sheet in the Neogene. A relatively weak ACC, initiated during the Eocene39, intensified in the late Oligocene ~26 Ma40 but modern-like strengths only developed in the late Neogene41. The development and latitudinal position of the fronts associated to the ACC are, however, still poorly constrained. Meanwhile, a long-term trend of BWT change and how the oceanic processes are coupled to Antarctic ice dynamic is still unclear. To shed light on the links between (Antarctic) ice volume and dynamics, Southern Ocean oceanography and latitudinal SST gradients, we present a detailed reconstruction of Neogene STF migration history and surface and bottom water temperature offshore Tasmania, and pair these with estimates of Antarctic ice volume change from the MCO across the MMCT. We use dinoflagellate cyst (dinocyst) biogeography42 to reconstruct the position of Southern Ocean currents and fronts and combine these with published SST reconstructions27. Finally, from benthic foraminiferal Δ47, we assess deep-water temperature changes at Ocean Drilling Program (ODP) Site 1168, as well as seawater δ18O (δ18Osw) as a proxy for Antarctic ice change.

In this study, we demonstrate that there is a strengthening and equatorward migration of the STF from ~53° to ~42° between ~14 Ma and 7 Ma, concurrent with progressive sea surface and bottom water cooling. The deep ocean cooling can completely explain benthic foraminifer δ18O evolution, implying stable global ice volume. After 7 Ma, the northwards shift of the STF is limited by the Australian continent, even though the SSTs continue to decrease. To reconcile expansion of subpolar ocean conditions and progressive Neogene Southern Ocean cooling with stable ice volume and compelling evidence of ice advance, we argue that the Miocene Antarctic ice sheet progressively lowered in height while expanding seawards during the mid-Miocene. We present idealised ice sheet model simulations that physically constrain this hypothesis. This changed geometry induced strong regional oceanographic responses with expansion of sea ice, cooling of the region of bottom-water formation and northwards migration of ocean fronts.

Results
Dinoflagellate-based surface oceanographic reconstruction of the subtropical front
The vast majority of the dinocysts encountered in the Neogene sediments from ODP Site 1168 are extant species of the modern Southern Ocean. The use of inferences from modern biogeographic distributions and affinities of dinocyst assemblage clusters42 (see “Methods” section) hence allows reliable reconstructions of paleoceanographic conditions.

In early Miocene sediments at Site 1168, dinocyst assemblages are dominated by warm/temperate Spiniferites spp. (Fig. 2c). This assemblage resembles the Spin-cluster of ref. 42 (Fig. 2b), which now mainly thrives along the northwest coast of Australia and in low latitudes in the eastern Indian Ocean42. This cluster is associated with a modern SST of ~29 ± 0.5 °C, a temperature in line with that derived from biomarkers (Fig. 2a). Early Miocene SSTs at Site 1168 were ~13 ± 6 °C (calibration error) warmer than today based on biomarkers, in spite of a ~10° more poleward position of the site43. Given these SSTs and dinocyst assemblages, we infer a strong influence of the (proto-) Leeuwin Current, delivering heat and sustaining low-latitude dinoflagellate assemblages from western Australia towards the site. It implies that the STF was located to the south of the site. Gradual increases of Operculodinium spp. in this interval suggests gradually cooler-water influence, with an approaching STF from the south. We find occasional northward migrations of the STF (e.g., at ~22 Ma) in sporadic abundance of N. labyrinthus, concomitant to SST cooling (Fig. 2c).

Dinocysts are poorly preserved in MCO sediments (Fig. 2c), and Glycerol Dialkyl Glycerol Tetraethers concentrations are low27, pointing to enhanced sediment oxidation44. The available palynological data for the MCO shows that the Spin cluster was replaced by Impagidinium paradoxum and I. patulum, which in the modern are restricted to temperate to equatorial open ocean regions between sub-tropical and subpolar systems45. Although it is unclear how this dinocyst assemblage differs from the Spin cluster in terms of ocean temperature, the biomarker-based SSTs indicate continued warmth during the MCO at the site (Fig. 2). In any case, Site 1168 remained north of the STF.

The mid Miocene Climatic Transition (MMCT, ~14.5 to 12 Ma) marks the first interval of prevailing N. labyrinthus (Fig. 2b, c). This species (Nlab cluster in ref. 42) is found most abundant in sediments south of the STF, in the modern subantarctic zone. We interpret the proliferation of Nlab and a progressive cooling towards subantarctic zone-like conditions (Fig. 2) as a northward migration of the STF. At MMCT, the STF reached a similar position relative to that of Australia as during the last glacial maximum46–48 (Fig. 3). Subsequent high-amplitude, short-term fluctuations of dinocyst assemblages between the Nlab- and Spin-cluster, and, albeit less pronounced, biomarker-based SSTs (Fig. 2a), indicate strong (SSTs between 29 °C and 11 °C; Fig. 2c) variability of the latitudinal position of the STF until 7 Ma (Fig. 2).

From ~7 Ma, N. labyrinthus started to decrease in abundance. We interpret a southward migration of the frontal systems relative to Australia from the decline in Nlab and the return of north-of-STF clusters. Apparently, this continued cooling is not directly related to continued shifts of ocean frontal systems, but a cooling of the STF itself and tectonic drift. The Nlab-Cluster and the slightly warmer high-Ocen-Cluster alternate in the Pliocene (4.5–2.5 Ma) on orbital time-scales (Fig. 2b, c), which is close to the modern assemblage, and bracket the modern STF42 (SSTs between 25 °C and 10 °C; Fig. 2a). The Pliocene stands out as a warmer interval than the end of the Miocene, from both the biomarker-based SSTs (~15 °C)27 and the dinocysts assemblages. In addition, the Pliocene yields much more abundant O. centrocarpum and I. aculeatum than the rest of the record, which means that the STF was much closer to ODP Site 1168 in the Pliocene than in the early Miocene, when Spiniferites dominated.

Overall, we deduce long-term cooling from the dinocyst assemblages, despite the ~8° northward tectonic movement of the site during the Neogene. There was strong variability over glacial-interglacial climate fluctuations. The STF moved gradually northwards during 22–7 Ma from ~53°S to 42°S (Fig. 3a–c). We infer a concomitant strengthening of the STF from steepened latitudinal SST gradient among mid latitudes27, and from the fact that the STF was progressively pushed towards the southern margin of the Australian continent. From 7 to 2.5 Ma, the STF moved south from the site again, likely because of Australia’s continued northward drift. This allowed for the return of influence of the warm (proto-) Leeuwin Current at the Site (Fig. 3c, d).

Benthic foraminiferal stable isotope ratios, Δ47 and sea water δ18O
The δ18Obf and δ13Cbf records generally follow trends recorded at other Southern Ocean sites36,37 (Fig. 4), including a 1‰ negative offset in δ18Obf compared to the CENOGRID compilation18. At ~10 Ma, δ18Obf gradually increases from 1.5‰ (MCO) to 2.5‰, followed by a further rise to ~2.8‰ at the end of the Miocene (~5.3 Ma). Remarkably, we do not record pronounced steps across the MMCT as seen in other records49. The pronounced δ13Cbf maxima (from 17 Ma) likely reflects the Monterey carbon isotope excursion50–52 and values are in line with those in other records.

The benthic foraminiferal clumped isotope data from Site 1168 fill critical mid- and late-Miocene gaps in existing BWT compilations38 and thus Antarctic ice dynamics (Fig. 5a). BWT, based on Δ47 data in ~1 Myr bins, at Site 1168 decreased gradually from 9.9 ± 4.0 °C (95% confidence interval) in the MCO (17–14.5 Ma) to 5.0 ± 2.5 °C around 10–9 Ma (Supplementary Data 1 and Fig. 5a). While the decreasing trend in mid-Miocene BWT is evident, the confidence intervals on the individual data points leave ambiguity on the significance of the point-to-point cooling. A Student’s t-test on the bins, however, proves a significant difference in Δ47 between the MCO (17–14.5 Ma) and late Miocene (10–9 Ma; p = 0.02; Table S1). Hence, the BWT cooling from the MCO to 9 Ma is significant. The ~8 °C data point at ~8 Ma has only 23 replicates and the longest binned time interval, and because of the resulting high uncertainty we leave this data point out of our interpretations (Fig. S1). By the end of the Miocene (5 Ma), BWTs were slightly elevated (5–6 ± 3 °C) compared to the mid-late Miocene.

Previous studies have pointed out the unexpected warmth of mid-Miocene BWTs in their reconstructions and discussed potential but undiscernible biases on Δ47-based BWT from recrystallisation and pH36–38,53. Since benthic foraminifera at Site 1168 are well preserved (Figs. S2 and S3), and seawater chemistry, dissolution and recrystallisation have very limited influence on benthic foraminifera Δ47 composition54,55, we consider our BWT reconstructions reliable and confirm from Site 1168 the previous inferences of much warmer BWTs in the Miocene than present. By applying a new calibration56, the BWT shifts by around ~2.4 °C towards lower values (Fig. S4) and results in a smaller global ice volume, but the amplitude of changes remains the same for both BWT and δ18Osw (see “Methods” section).

Methods
Site description
ODP Site 1168 (42°36.5809′S; 144°24.7620′E; 2463m modern water depth) (Fig. 1) is located on the continental slope of the west-Tasmanian continental margin, with a modern seafloor temperature of 2.5 °C92. The site sits on the northern edge of the Subtropical Convergence zone, which separates warm, saline subtropical waters from comparably cold and fresh subantarctic water masses93. During the Neogene, the location of Site 1168 tectonically drifted along with Tasmania and Australia from 52°S at 23 Ma to its modern position at 42°S94. The Neogene bathymetry was lower bathyal/upper abyssal (1000–2500m), midway on the continental slope92. During this northward tectonic drift, the Southern margin of Australia was continuously bathed by the eastward flowing (proto-) Leeuwin Current40,95. Hence, Site 1168 is well-suited to study the Neogene evolution of the STF. We applied the same age model for the sediments as in Hou et al.27. (Fig. S5).

Palynology
We studied 131 samples for palynological content96. The processing of sedimentary samples for palynological analysis followed standard procedures at the GeoLab of Utrecht University97. Dried sediment samples were crushed and weighed (on average 10g, standard deviation, SD, of <1g) before they were dissolved with 30% hydrochloric acid (HCl) and 38% hydrofluoric acid (HF) for carbonate and silicate removal, respectively. The remaining palynological residues were sieved on a 10 μm nylon mesh, using an ultrasonic bath to disintegrate agglutinated organic particles. The palynological residues were mounted on glass slides using glycerine, sealed, and counted (under 200 and 400 magnification) using an Olympus CX41 optical microscope. When possible, at least 200 dinocyst specimens were counted98. Samples containing less than (including) 50 dinocyst specimens were excluded for further analysis and interpretation.

We further applied the model of Thöle et al.42. (Fig. S6) to infer paleoceanographic conditions from dinocyst assemblages. Specifically, we inferred the 25–75% SST ranges of the clusters in Thöle et al.42. that the downcore assemblages compared most to (Fig. 2).

Foraminiferal preparation
Each sediment sample was freeze-dried, washed over a 63 μm sieve, oven-dried at 50 °C and then dry-sieved into different size fractions. We mainly picked tests of Cibicidoides mundulus from the 250–355 μm size fraction for our measurements. We cracked open the picked specimens and ultrasonicated the test fragments in deionized water (3*30s) to remove adhering sediment, organic lining and nannofossils. The test fragments were dried at room temperature overnight. In order to obtain enough material, other benthic species are also processed. We use Cibicidoides mundulus and Cibicidoides (Planulina) wuellerstorfi for both stable and clumped isotopes analyses. Data from other benthic or infaunal species Pyrgo sp., Gyroidina soldanii, Uvigerina peregerina are only used for clumped isotopes99 (Fig. S2).

Clumped isotope analysis
Clumped isotope measurements were performed using Thermo Scientific MAT 253 and 253 Plus mass spectrometers at the GeoLab of Utrecht University. Both mass spectrometers were coupled to Thermo Fisher Scientific Kiel IV carbonate preparation devices. CO2 gas was extracted from carbonate samples with phosphoric acid at a reaction temperature of 70 °C. A Porapak trap included in each Kiel IV carbonate preparation system was kept at 120 °C to remove organic contaminants from the sample gas. Between each run, the Porapak trap was heated at 120 °C for at least 1h for cleaning. Every measurement run included a similar number of samples and carbonate standards100. In all, 3 carbonate standards (ETH-1, 2, 3) with different δ13C, δ18O and Δ47 compositions and ordering states were used for monitoring and correction of the results101. Two additional reference standards (IAEA-C2 and Merck) were measured in each run to monitor the long-term reproducibility and stability of the instrument. We achieve the necessary precision by averaging ~30 clumped isotope values measured on small (70–100 μg) carbonate samples101–104. External reproducibility (1 standard deviation) in Δ47 of IAEA-C2 after correction was 0.033‰. The δ13C and δ18O values (reported relative to the VPDB scale) of IAEA-C2 showed an external reproducibility (1 standard deviation) of 0.18‰ and 0.21‰, respectively105.

Deep sea temperature and δ18Osw calculation
We converted the sample Δ47 values (averages over ~30 separate measurements each) into temperature (T, in °C) using a calibration based on various recent datasets from core-top-derived foraminifera, corrected with the same carbonate standards as used in our study62:
T = (0.0431 * 10^6 / (Δ47 - 0.1876))^0.5 - 273.15 (1)

Δ47-based BWTs were used in combination with δ18Obf to calculate δ18Osw (reported relative to VSMOW) with Eq. (9) of Marchitto et al.106:
δ18Obf (VPDB) - δ18Osw (VSMOW) + 0.27 = (-0.245 ± 0.005) * T + (0.0011 ± 0.0002) * T^2 + (3.58 ± 0.02) (2)

For these calculations, δ18Obf values of the genus Cibicidoides were averaged over the same intervals as have been used for Δ47 averaging. Calibration uncertainties and measurement error were addressed by applying error propagation. The Meinicke et al.62,107,108] calibration error was propagated using the R package (clumpedcalib)109 that utilised a bootstrapped York regression slope-intercept pairs to the bootstrapped mean values for each bin. It should be noted that the calibration error is very small compared to the analytical error. The new calibration56 excludes benthic foraminifera-based data and it shifts the BWTs parallelly ~2.4 °C colder. As a result, the magnitude of cooling and relative ice volume changes are unaffected (Fig. S4). Since we are using benthic foraminifera as substrates, we decide to keep using the Meinicke calibration.

Ice sheet modelling
To demonstrate the viability of a precipitation regime change leading to a fundamentally different volume-to-area ratio of the Antarctic ice sheet, we deploy the 3D thermodynamical ice sheet/shelf model IMAU-ICE v1.1.1110,111. In the standard set-up90, climate forcing follows from pre-run warm and cold snapshot climate simulations112. The applied climate forcing is transiently calculated based on the prescribed CO2 concentration and the modelled ice sheet size, through a matrix interpolation method110. Sea ice is included in the climate model forcing, but a dynamical response is not calculated by IMAU-ICE. Equilibrium experiments are performed at various CO2 levels between preindustrial and 3x preindustrial CO2 values, with insolation at present-day levels and initiated from an ice-free Miocene Antarctic topography113. Here, we perform additional sensitivity experiments, in which we apply a fixed precipitation increase and enhanced sub-shelf melt rates. The precipitation anomaly is calculated as 25% of the warm snapshot precipitation fields, sub-shelf melt rates are set to 400m/yr114,115.

These sensitivity experiments yield large-scale glaciation at a higher CO2 level (Fig. 6a), and an overall increased volume-to-area ratio (Fig. 6b). Notably, simultaneously reducing the CO2 level from 504 to 394 ppm and removing the anomalous forcing, leads to significantly larger ice sheet area, while the interior ice sheet height is severely reduced (Fig. 6c and S7). These idealised experiments exemplify our hypothesised Antarctic ice sheet change at the MMCT.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Hou et al NCOMMS 2023\nThe new manuscript by Hou et al investigates the way in which the Southern Ocean evolved through\nthe Miocene. Specifically, they explore the transient migration of oceanic fronts during the long-term\ncooling phase from the Miocene Climatic Optimum through the Mid Miocene Climate Transition and\ninto the Late Miocene, using multiple new proxy datasets and analyses. These reconstructions reveal\npalaeoenvironmental changes that differ substantially from previous work, and so to investigate the\ndrivers of the inferred changes, Hou et al present a series of ice sheet model simulations. Overall, they\nfind that deep water isotopic anomalies during this time, instead of being partitioned 50/50 into\ntemperature and ice volume changes as has previously been suggested, can in fact be almost entirely\nexplained as temperature. This means that ice volume (and by implication sea level) reconstructions\nneed to be reassessed.\nOverall I found the paper to be very well written and very well illustrated. It is extremely readable and\ninteresting, the data and methods are logically and objectively presented, and the arguments are\nclearly explained. I found the study to be remarkably thorough and convincing. In fact, I struggled to\nfind anything of any substance to comment on - my only comments are very minor indeed:\nline 32: 'On' should be 'In'\nline 35: 'driven by westerlies and bathymetry' - I know what is meant here, but I wonder whether\nbathymetry really 'drives' ocean currents? Maybe 'steered' or 'deflected' might be better?\nFigure 2: It could be useful to highlight along the x axis periods like the MCO and MMCT that are\nmentioned in the text, just for ease of reference.\nline 221: I think 'BTW' should be 'BWT'\nline 301: 'enhanced ice-rafted debris off east Antarctica89 (Fig. 5c)' - I couldn't find in 5c which of the\nplotted records this refers to?\nline 302: 'this part of the Miocene' - I assume the authors mean the period 6-9 Ma as mentioned\nearlier in the paragraph - maybe clarify, or use a term like 'Late Miocene' with an indicator on Fig. 2?\nline 318: 'Taken the above together' --> 'Taking...'\nAll in all an excellent and important piece of work, and one highly deserving of publication in Nature\nCommunications.\nN.R. Golledge 7th Aug 2023", "reviewer_2": "The manuscript “Reconciling equatorward migration of Southern Ocean fronts with minor Antarctic ice\nvolume change during the Miocene cooling” by Hou et al. presents a combination of dinoflagellate and\nforaminifera-based sea water temperatures with ice sheet modeling in an attempt to solve an\napparent controversy in the Miocene climate history around Antarctica focused on the latitudinal\nmigration of the oceanic front systems, specifically the subtropical front (STF). Traditionally, the Mid-\nMiocene climate transition (MMCT) at ~14 Ma has always been regarded as the glaciation of an\nimportant part of Antarctica as expressed in benthic oxygen isotope records. This transition occurred\nwithin in a few orbital cycles. The reconstructions presented here though suggest that most of the ice\nvolume change as based on the benthic d18O records may rather have been a cooling of deepwater\ntemperatures, using clumped isotopes. This cooling occurred in parallel with a northward moving of\nthe STF as shown by assemblages of dinoflagellates and their reconstructed sea surface temperatures.\nThe ice sheet modeling indicates that even though ice volume itself may not have changed very much,\nmajor changes in the ocean-ice interaction with the resulting oceanic cooling did occur due to\nchanging geometry of the ice sheet.\nThis study presents a very interesting reconstruction and hypothesis on (global) climate development\nthrough the middle-late Miocene, a time period that actually has not received as much attention as the\nmid-Miocene climate optimum and the MMCT. The combination of different proxy approaches, i.e.\ndinoflagellate assemblage and SSTs, deep water temperatures based on clumped isotopes, and ice\nsheet modeling, provides a robust reconstruction with interesting consequences for future climate\nreconstructions during the Miocene, and possibly even for future climate scenarios.\nI do not have major criticisms on this manuscript, but rather some points of interest that may need\nsome clarification or can be included. The manuscript is well-written and very clear to follow and will\nbe a fitting contribution to Nature Communications after minor revisions have been made.\nComments:\nAlthough I am not a modeler, the first and main comment is that it would be very nice to see the\neffect of the modeling on the actual sea ice extent. Currently the changes are assumed to happen and\nsome proxy studies do seem to support this, but a direct link from the model between the changing\ngeometry and sea ice occurrence would be helpful. And extending this, the logical next step would\nof course be to see its impact then on the Westerlies and the oceanic fronts.\nIt would have been nice and supporting the hypothesis to have a reconstruction that is including a\nhigh-resolution interval directly at the MMCO, i.e. the time interval where the benthic d18O record is\nshowing the largest changes and always has been interpreted as representing ice volume changes.\nWhichever signal is in the d18Obenthic record, it did occur within a few orbital cycles, so what was the\ntrigger to make the ice sheet change its geometry, i.e. did this also occur relatively fast, with both a\nquick response (d18Obenthic) but also keep influencing conditions for the rest of the Miocene?\nAlthough not the main topic of this manuscript, the data is still presented, so I am curious about the\nincrease in O. centrocarpum and I. aculeatum near the intensification of Northern Hemisphere\nGlaciation at the end of the Pliocene (Fig. 2). Does this imply a southward migration of the STF again,\neven though this may be counter-intuitive?\nMinor comments/typos:\nLine 39: change on to in\nLine 79-80: I would rephrase this; it currently reads as if the STF reaches all the way to the bottom\n(2400 m).\nLine 83: MA > Ma\nLine 103: remove in\nLines 112-115: check the order of the figures; currently it is Fig. 1, 2, 5, 4, 3.\nLine 133: I assume the N. labyrinthus equals the Nematosphaeropsis panel in figure 2?\nLine 165: The Christensen (2021) shows that at some stage in the late Miocene warm subtropical\nwaters may actually have come from the eastside of Australia after Australia moved far enough to the\nnorth. The presence of the Leeuwin Current during the Miocene is not completely sure. Some studies\nindicate that it may not have been formed until the Pliocene related to changes in the Indonesian\nThroughflow on which it is fully dependent (e.g. Gallagher et al., 2009).\nLine 321: BTW > BWT\nSample preparation: the foraminifera were not cleaned following trace metal procedures (e.g. Barker\net al., 2003)?\nLine 368: Has the use of Pyrgo been tested as reliable for clumped isotopes? It is often included\nbecause of its size, but has also been shown as having anomalous geochemical signatures in some\nsettings. Specifically, radiocarbon dating of Pyrgo sp. often gives much older results possibly related to\nPyrgo incorporating older, bioturbated material.", "reviewer_3": "Hou et al. aim to reconstruct the Miocene subtropical front migration and ice-ocean interaction off\nTasmania, using a state-of-the art multi-proxy approach, combining micropaleontology (with\ndinoflagellate abundance), and isotopic measurements (bottom water temperature with clumped isotope\nand the global ice volume by combing both clumped and stable isotopes). Thanks to the novel\nclumped isotope dataset combined with other temperature data, the authors found cooling sea surface\nand bottom temperatures (SST and BWT, respectively), with evidence of stable ice volume and ice\nexpansion, as confirmed by the model. This is an interesting hypothesis that deserves to be published.\nThe manuscript is well-written with data-driven interpretations and properly referenced. However, I\nhave a few comments.\nWhile reading, I was missing a discussion of SST proxies and their comparison. The authors used 3\ndifferent methods and if I understand correctly two were already published (biomarkers - Some\nreferences are missing, for example line 77). In the dinoflagellate section, the authors repeatedly\nmention SST without systematically referring to proxies. I would also suggest writing the uncertainties\nfor all temperatures mentioned in the text.\nIn addition, I would suggest adding a summary figure with your main results from both isotopic and\nmicropaleontologic data along with the other already published data listed in the discussion such as\nthe IRD. The figure 5 is referred before figures 3 and 4 in the text, emphasizing the importance to\npresent a global figure showing changes in key species/groups of dinoflagellates (STF) and\ntemperatures.\nAt the end of the abstract, the authors mention conditions specific to their hypothesis related to\nprecipitation patterns and ice-ocean interaction. This sentence is quite broad. I would suggest being\nmore specific.\nIn the introduction, I would suggest writing a clear paragraph with the objectives. As it stands, the\nobjectives are mixed up in a paragraph (lines 69-80), making it difficult to read them.\nIn the dinoflagellates part, (lines 146-147), I am not to get how the authors conclude this\ninterpretation, can they clarify?\nIn the Figure 3: Are black solid lines (STF) based on a model or drawn based on data? If it's based on\ndata and drawing hypothetically, I would suggest using dashed lines, especially for the southwest\ncoast of Australia.\nIn the method, the Δ47 (47) must be in subscript. Also, the authors used a combination\nof different benthic species for the clumped isotope amalyses, including Pygo sp., little used in\npaleoceanography and which can potentially build their shells differently from species classically used\nin paleoceanography. Have you checked for any differences between the species mix and a mono-\nspecies or between classic species and Pygo sp.?"}, "author_response": "Response to reviewer: We thank the reviewer for the constructive response upon reading our\nmanuscript.\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: rephrased to “and steered by bathymetry.”\nChanges made in the manuscript: We added 2 bars reflecting the MCO and MMCT time intervals to\nFigure 2\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: It was supposed to be Fig. 5d. We have now incorporated IRD\nvisualization into Fig. 5 as suggested by reviewer 3.\nChanges made in the manuscript: rephrased to “the latest Miocene”\nChanges made in the manuscript: corrected.\nResponse to reviewer: We thank the reviewer again for the positive assessment.\nResponse to reviewer: We thank the reviewer for their positive assessment of the manuscript and its\nfitness for Nature communications. We respond to the thorough comments in detail below.\nResponse to reviewer: Sea ice modelling is indeed of great interest to not only the modelling community\nbut also the paleocommunity. However, the ice sheet model IMAU-ICE that we used in our study can\nnot resolve/simulate sea ice extent. We do not model sea ice with the ice sheet model. There is sea ice in\nthe climate model simulations that isused as forcing, but this can not dynamically respond to the changing\nice sheet conditions we simulate.\nChanges made in the manuscript: We added “Sea ice extent is not included in the IMAU-ICE model.”\nin the method, ice sheet modelling section.\nResponse to reviewer: We appreciate the point proposed by the reviewer. However, the sampling\nresolution in this study is not high enough to answer the question, and analysing clumped isotopes in\nhigh temporal resolution is complicated because of the necessity to run numerous replicates to achieve\nlow enough uncertainties on the reconstructions. Clearly higher-resolution work is required as follow-\nup, but this has to be performed on expanded sections where accumulation rates of benthic foraminifer\nis highest. Previous studies (Modestou et al., 2020; Leutert et al., 2021) have indicated that Δ -based\nChanges made in the manuscript: No changes made.\nResponse to reviewer: The increase in warm species in the Pliocene is likely due to the position of the\nAustralian continent, which by then approaches its modern position. Meanwhile, there are high amplitude\nwarm/cool species alternations on orbital timescales. We are preparing another manuscript with higher\nsampling resolution focusing on the late Pliocene, including the M2 and mid-Piacenzian Warm Period,\nand are working on higher-resolution data for the Pleistocene as well. As the Miocene long-term trends\nwere the focus of this study, we decided not to spend any further explanation on it in this paper.\nChanges made in the manuscript: We added a sentence in the results stressing the return of warmth in\nthe Pliocene compared to the late Miocene, but in the interest of later publications, we refrain from going\ninto detail on this further. We added “The Pliocene stands out as a warmer interval than the end of the\nMiocene, from both the biomarker-based SSTs (~15 °C) (Hou et al., 2023) and the dinocysts\nassemblages.”in line 218.\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: rephrased to “...temperature changes at Ocean Drilling Program Site\n1168, ....”\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: corrected.\nChanges made in the manuscript: We use “Nematosphaeropsis labyrinthus” in Fig. 2 now.\nResponse to reviewer: The Tasman Leakage suggested by Christensen et al. was a subsurface watermass,\nlikely flowing well below the surface but also well above the bottom of our site, therefore it remains a\nquestion how much surface warming it caused. The definition of the Leeuwin Current is rather vague,\naccording to Jackson et al. (2019), so it remains somewhat open what one would interpret as Leeuwin-\ncurrent influence. However, we agree with the reviewer that imposing the terminology of modern ocean\ncurrents to paleosettings have a risk of overinterpretation.\nChanges made in the manuscript: We use “(proto-) Leeuwin Current” instead throughout the\nmanuscript.\nChanges made in the manuscript: corrected.\nResponse to reviewer: We followed the standard procedure for cleaning foraminifera prior to clumped\nisotope analyses, which was also used in Agterhuis et al. (2022) and Meckler et al. (2022) for instance.\nThe advantage of this is that results can be easily compared when a consistent methodology was used.\nChanges made in the manuscript: No specific changes made.\nResponse to reviewer: We appreciate the concern raised by the reviewer. The conclusions that there are\nno vital effect offsets of benthic foraminifera for clumped isotopes and there are no discernible species-\nspecific offsets were actually drawn by using Pyrgo sp. (Piasecki et al., 2019).\nChanges made in the manuscript: We now cite Piasecki et al. (2019) in the manuscript, method section.\nResponse to reviewer: We are grateful to reviewer for the positive feedback and appreciation of our\nnovel hypotheses.We reply on the comments in detail below.\nChanges made in the manuscript: no specific changes made.\nResponse to reviewer: Discussion about the biomarker-based SSTs is indeed relatively missing as they\nwere already published and discussed in Hou et al. (2023), by the same first author.\nChanges made in the manuscript: We now cite Hou et al. (2023) in line 78. We also now mention the\nsources and error of SST where needed.\nResponse to reviewer: We thank the reviewer for the notice. We intended Fig. 5 to represent the new\ndata generated in this study. But we will incorporate IRD data into this figure.\nChanges made in the manuscript: We moved biomarker-based SSTs to Fig. 2 and incorporated\npreviously published IRD data in Fig. 5.\nChanges made in the manuscript: We rephrased to “...precipitation regimes on Antarctica and ice-\ninduced ocean cooling and requires rethinking the interactions between ice, ocean and climate.”\nResponse to reviewer: We thank the reviewer for the suggestion.\nChanges made in the manuscript: We now slightly re-structure the paragraph. “...The development\nand latitudinal position of the fronts associated to the ACC are, however, still poorly constrained.\nMeanwhile, a long-term trend of BWT change and how the oceanic processes are coupled to Antarctic\nice dynamic is still unclear. To shed light on...”\nResponse to reviewer: During 7–5 Ma, we observed the decline of N. labyrinthus and an increase of\nSpiniferites spp., thus, we infer that the STF “moved” southward relative to Australia. Meanwhile,\nbiomarker-based SST dropped. These are likely a combined effect of both surface cooling and tectonic\ndrift.\nChanges made in the manuscript: we elaborated more on this, to avoid confusion as follow “... We\ninterpret a southward migration of the frontal systems relative to Australia from the decline in Nlab\nand the return of north-of STF clusters. Apparently, this continued cooling is not directly related to\ncontinued shifts of ocean frontal systems, but a cooling of the STF itself and tectonic drift.”\nResponse to reviewer: The solid lines of STF are based on data and drawn hypothetically.\nChanges made in the manuscript: We use now dashed lines for the southwest coast of Australia when\nthere are less data supporting. We also added “Positions of the STF are hypothetically drawn based on\nthis study and refs (Orsi et al., 1995; Groeneveld et al., 2017)”\nResponse to reviewer: Please see the response to the last comment of Reviewer 2\nChanges made in the manuscript: We corrected the subscript in line 383 and checked the manuscript\nthroughout for consistency."}, {"round": 2, "reviewer_comments": {"reviewer_2": "This a response to the revision of the manuscript by Hou et al. This is my second review of the\nmanuscript after an already very positive review in the first round. The comments and suggestions\nI made were not all included but still satisfactorily responded to.\nFollowing this I only would like to come back to my comment on the Pliocene increase in O.\ncentrocarpum. The authors point out that an upcoming study will go into higher resolution to\ndiscuss the Pliocene, and because it is not the main focus of this manuscript, they do not further\ndiscuss the increase in Ocen. However, the increase is larger than any change in Ocen during the\nMiocene, so it really pulls the eye. So, in that case I would suggest to just leave the Pliocene part\nout of the dataset; it is only a small part of the record, and would get its own attention in the\nupcoming manuscript.\nFor the rest I have no further comments and recommend this manuscript for publication in Nat.\nComm.\nJeroen Groeneveld", "reviewer_3": "Hou et al. have included all reviewer comments in this revised manuscript. The article is well written\nand the arguments are well presented, supported by data. I only have one comment regarding\nthe clumped isotope method. A new calibration was released last week (Daeron and Gray, 2023),\nrecalculating the foraminiferal calcification temperatures for the clumped isotope calibration. As a\nresult, the clumped isotope derived temperatures are colder than those obtained with the “old”\ncalibration. Another consequence would be a change in the d18Osw reconstruction, with more\nnegative values. I strongly encourage the authors to recalculate the clumped isotopes derived\ntemperature using the new calibration, to check if their hypothesis still holds with the new values\nof d18Osw.\nDaëron, M., & Gray, W. R. (2023). Revisiting oxygen‐18 and clumped isotopes in planktic and\nbenthic foraminifera. Paleoceanography and Paleoclimatology, e2023PA004660."}, "author_response": "Response to the reviewer: We thank the reviewer for the positive comments on our manuscript.\nThe upcoming Pliocene manuscript focuses on high resolution for the interval 2.9–3.5 Ma, so we\ndo agree that the long-term changes in Pliocene dinocysts assemblages fits this manuscript.\nChanges made in the manuscript: We revised Figure 2 to better illustrate the Nlab/High-Ocen\nalternations. We added more context in the main text: “In addition, the Pliocene yields much more\nabundant O.centrocarpum and I. aculeatum than the rest of the record, which means that the STF\nwas much closer to ODP Site 1168 in the Pliocene than in the early Miocene, when Spiniferites\ndominated.” in line 156.\nResponse to the reviewer: We thank the reviewer for the positive comments on our revised\nmanuscript. Applying the calibration (equation 5 in Daëron and Gray, 2023) to our data, the BWT\nshift by around ~2.4°C towards lower values (see the figure below), but are within the 95%\nconfidence intervals of our originally reported values. Since the overall trends in the dataset remain\nunaffected our main conclusions regarding BWT changes still hold. As the reviewer points out\ncorrectly, the reconstructed d18Osw values will do shift towards more negative values as well when\nusing the new calibration and this would in turn point to a smaller global ice volume during the\ntime interval of interest. However, determination of the absolute ice volume is not subject of our\nmanuscript as we instead focus our discussion on relative ice volume changes, that are unaffected\nby the calibration we choose. For the main part of the manuscript, we keep the calibration of\nMeinicke et al. that we also applied in our original submission. Previously published clumped BWT\ndataset that we use for comparison in our manuscript use the same calibration. Another reason is\nthat the calibration developed by Daëron and Gray (2023) excludes benthic foraminiferal based\ndata.\nChanges made in the manuscript: We added the comparison figure below to the supplementary\ninformation and made comments on the Daëron and Gray (2023) calibration in the main text.\nWe added to the main text “By applying a new calibration (Daëron and Gray, 2023), the BWT shifts\nby around ~2.4°C towards lower values (Fig. S4) and results in a smaller global ice volume, but the\namplitude of changes remains the same for both BWT and δ18Osw.”in line 227;\n“An alternative calibration will only influence the absolute values of BWT and δ18Osw. The\nmagnitude of cooling and relative ice volume changes are unaffected” in line 440.\nWe rephrased line 257–259 as “Nevertheless, the inconsistence between Δ47‐based absolute\nδ18Osw (therefore ice volume) and far‐field sea level reconstructions is a general arising question\ndependent on calibrations67,78 and requires further exploration38,79,80.”"}]

Decision letter