Episodic intensification of marine phosphorus burial over the last 80 million years

Domain: earth_science
Published: 2024-08-28
Editorial outcome: major_revision

Abstract
Marine phosphatization events cause episodic carbonate fluorapatite (CFA) precipitation on seamounts, and are commonly linked to growth hiatuses in ferromanganese (Fe-Mn) crusts. However, the complete record of these events and their paleoenvironmental significance remains poorly understood, in large part due to poor age constraints. Here, we apply U-Pb dating to CFA in Fe-Mn crusts from Western Pacific seamounts. These data exhibit good alignment with Sr isotope ages, revealing six potential phosphatization events. This established CFA chronology tightens the timespan of phosphatization events and refines the age framework of Fe-Mn crusts. We subsequently utilize a multiproxy approach to demonstrate that the phosphatization events occurred coeval with the expansion of oceanic oxygen minimum zones. The Western Pacific Fe-Mn crusts thus document major perturbations in global oceanic phosphorus cycling, which appear to have been driven by climate-induced increases in primary productivity linked to changes in global ocean circulation.

Introduction
Phosphorus (P) is the dominant limiting nutrient for oceanic primary productivity on geological timescales1, and thus the long-term bioavailability of P influences atmospheric carbon dioxide levels and climate via changes in organic carbon burial2,3. The P content of the ocean is determined by a balance between the supply of P, which in a well-oxygenated ocean is mostly from continental weathering, and the loss of P via burial in marine sediments4,5. Dissolved oceanic P is dominantly delivered to the sediments in association with organic matter and Fe (oxyhydr)oxide minerals6. During diagenesis, however, sink-switching to authigenic phases commonly occurs, with carbonate fluorapatite (CFA) being a major sink for P7. CFA often precipitates in the voids or veins of ferromanganese (Fe-Mn) crusts on seamounts8,9, with periods of enhanced CFA precipitation being termed ‘phosphatization events’.

Carbonate fluorapatite in Fe-Mn crusts can provide valuable insight into Cretaceous to modern paleoclimatic conditions, oceanic paleo-redox variability, and P cycle dynamics10,11. In particular, the formation of seamount CFA is considered to be closely linked to the expansion of oxygen minimum zones (OMZs)12. However, constraints on the onset and duration of expanded OMZ intervals are currently lacking, and thus an accurate absolute chronometer, such as U–Pb dating of CFA, is crucial to fully utilize the paleoenvironmental information recorded by CFA in Fe–Mn crusts. The oldest oceanic crust (dating back to the early Jurassic13) occurs in the Western Pacific (Fig. 1), providing a potential record of multiple intervals of enhanced CFA precipitation.

Phosphatization events are episodic and linked to hiatuses in Fe–Mn crust growth14, which are widespread across the world’s oceans8,11. One common way to detect growth disruptions in Fe–Mn crusts is by comparing their osmium (Os) isotope compositions with the seawater Os isotope curve15. However, the accuracy of this method is limited due to the low resolution of the seawater Os isotope curve16. Similarly, the application of strontium (Sr) isotope correlation methods to CFA does not yield absolute ages8 and is characterised by time intervals with limited variation in the 87Sr/86Sr ratio. By contrast, the apatite U–Pb chronometer is a precise dating method with wide-ranging applications in igneous, metamorphic and sedimentary systems17. Although U–Pb dating of sedimentary apatite is challenging, with some studies yielding U–Pb dates with either poor accuracy or younger than expected ages18–20, other studies have successfully dated CFA grains and bioclastic apatite from sedimentary rocks, yielding ages consistent with independent geological constraints21,22. Thus accurate U–Pb dating of CFA in Fe–Mn crusts would improve constraints on both the timing of phosphatization events and the growth history of Fe–Mn crusts.

In this study, we report in situ U–Pb dating of CFA using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) applied to fifteen samples from five Western Pacific Fe–Mn crusts (Supplementary Data 1; Supplementary Figs. 1 and 2). We utilize these CFA U–Pb ages (independently validated by the Sr isotope chronometer) to more precisely date periods of phosphatization during hiatuses in Fe–Mn crust growth, thus improving their age framework established using Os isotope chronometry. We also employ a variety of additional geochemical data to explore the processes that led to hiatuses in Fe–Mn crust growth and the onset of CFA formation to develop a comprehensive understanding of the environmental controls on oceanic seamount phosphatization events over the last 80 million years.

Results
U–Pb ages (Supplementary Data 2) were obtained from fifteen CFA veinlet/cavity infills, ranging from about 72 to 7 Ma. These ages fall into six distinct time periods that commonly coincide with an increased number of hiatus intervals in pelagic sediments from the Southeast Pacific (Fig. 2). The oldest and youngest of these CFA formation phases occur at 72–71 Ma and 8–7 Ma, respectively, with other intervals of CFA formation occurring at 56–54 Ma, 35–34 Ma, 25–23 Ma, and 11–10 Ma (Supplementary Table 1; Supplementary Fig. 3).

The corrected 187Os/188Os ratios of seventeen successive Fe–Mn oxide samples from the MCSD85 Fe–Mn crust drill core vary from 0.420 to 0.842, with an average uncertainty of 0.003 (2SE; Supplementary Data 3). The results align with the seawater Os isotope curve from ~78 to 7 Ma (Fig. 3). Notably, according to the Os isotope ages and sampling location, distinct hiatuses occurred at ~62–54 Ma, 51–42 Ma, 39–33 Ma, and 29–17 Ma.

In situ 87Sr/86Sr ratios of fifteen CFA samples vary from 0.70762 to 0.70901, with a mean uncertainty of 0.00014 (2SE). Based on the fitting of these results to the global seawater Sr isotope curve, the ages vary from ~75.6 to 5.6 Ma (Supplementary Fig. 4). In addition, the Sr isotope ratios of the calcite substrate in core MCSD85 range from 0.70740 to 0.70755, corresponding to an age older than 78 Ma.

Methods
Os isotope analyses
A total of 17 micro-drilled Fe–Mn oxide samples were taken from core MCSD85 (while carefully avoiding CFA), with a ~2 mm layer thickness and a powder weight of ~2 g per sample. The analyses of Os contents and isotopic compositions were conducted using N-TIMS (negative thermal ionization mass spectrometry) at the State Key Laboratory of Isotope Geochemistry (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences), following procedures76: (1) Sample powder (0.5–1 g) was digested in Carius tubes, and appropriate amounts of the individual 185Re and 190Os spike solutions were weighed and added; (2) The tubes were chilled in a liquid N2 and ethanol bath, and 2.5 ml of concentrated HCl and 7.5 ml of concentrated HNO3 were successively added. The Carius tubes were then sealed, steel jacketed, and heated in an oven at 230 °C for 24 h; (3) After decomposition, the glass tubes were again chilled in a liquid N2 and ethanol bath before opening. After thawing, the contents were poured into 20 ml centrifuge tubes to allow precipitation of the residual solids; (4) The supernatant solutions were transferred into 30 ml PFA (Perfluoroalkoxy) vials and subjected to Os solvent extraction by CCl4, followed by back-extraction into concentrated HBr77; (5) The extracted Os fraction was further purified by micro-distillation prior to N-TIMS analysis. Osmium was loaded in HBr on 99.999% Pt filaments (H. Cross Company, USA), and Ba(OH)2 emitter solution was loaded on top of the sample to enhance ion emission. Os isotope compositions were measured in pulse-ion counting electron multiplier mode. The results are listed in Supplementary Data 3.

Trace elements and U–Pb dating of CFA using LA-ICP-MS
Trace element analyses were conducted at the Key Laboratory of Marine Resources and Coastal Engineering, SYSU, using an Agilent 7900 ICP-MS coupled to a 193 nm Coherent Geolas Pro laser. For trace element concentrations in the Fe–Mn crusts and CFA, single spot ablations were performed with a 32–60 μm beam size, 5 J/cm2 fluence, and a 5 Hz repetition rate. The trace element compositions of Fe–Mn crusts were calibrated against the GSE-1G, NIST 610 and 612 standards, using Ti (EPMA data; Supplementary Data 4) as the internal elemental standard, while trace element analysis of CFA used P as the internal elemental standard. Trace element analyses of the crust and CFA were conducted separately to enhance the experimental precision, and the data for the CFA and Fe–Mn crusts are provided in Supplementary Data 5.

The U–Pb dating approach employed a line scan (raster) mode in order to obtain smoother signals and less downhole fractionation. To avoid Pb contamination and to improve the accuracy and precision of the U–Pb results, we pre-ablated the region of interest to be dated to remove material on the sample surface. A large spot size of 160 μm was used to maximise the signal intensity, and the analyses employed a 5 J/cm2 laser fluence, 5 Hz repetition rate, and a 2 μm/s scan speed. In order to ensure the reliability of the 206Pb/238U ratio, the oxide ratio in the plasma (as measured by ThO/Th) was limited to <0.5%. An internal smoothing device was employed throughout the experiments to ensure signal stability. Madagascar apatite 2 (MAD2; age = 474.25 ± 0.41 Ma) was used as the primary age reference material, with the Durango and McClure apatite used as secondary age reference materials78, with each analysis comprising a 20 s background measurement and a 45 s data acquisition (Supplementary Data 6).

Data reduction for U–Pb dating was conducted with Iolite 4.8.1 using the VizualAge_UcomPbined data reduction scheme79, which can account for variable common Pb in the primary apatite U–Pb age reference materials. The data reduction scheme first applies a common Pb correction to the user-selected age standard integrations, and then fits session-wide “model” U–Pb fractionation bias to the time-resolved U–Pb standard data. The final result output style is based on the PLASMA age recommendations (http://www.plasmage.org/recommendations/home.html). The U–Pb results, including the common Pb content (ƒ206c), were formatted using PlasmaAge.py code from GitHub (https://github.com/iolite-LA-ICP-MS/iolite4-python-examples). The IsoplotR software80 was used to construct the concordia/intercept diagram, isochrons, and for weighted mean age and uncertainty calculations. The 207Pb/206Pb initial ratios used to anchor the CFA U–Pb intercepts were obtained by Pb isotopic analysis of texturally adjacent Fe–Mn oxides (Supplementary Fig. 3; Supplementary Data 2).

LA-MC-ICP-MS strontium isotopic analysis
Strontium isotopic ratios of CFA were analyzed using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The laser system used was an Australian Scientific Instruments RESOlution LR coupled with a Nu Instruments Nu Plasma II MC-ICP-MS (Supplementary Data 6). A 193 nm ArF excimer laser was employed at a fluence of 4.5 J/cm2. Each measurement consisted of a 20 s background signal (gas blank) followed by a 40 s line scan using a 90 μm laser beam diameter at an 8 Hz repetition rate. Helium at a flow rate of 370 ml/min served as the carrier gas, which was mixed with argon (~0.97 L/min) via a T-connector before entering the ICP torch. The Nu Plasma II integration time was set to 0.5 s (equivalent to 80 cycles during the 40 s scan). Quality control was ensured by analysing standard apatite samples (including AP-1 and MAD2) after every ten samples81.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I found this to be an impressive manuscript that should be of broad interest. It presents results from a variety of geochemical and isotopic proxies to document the growth of Fe crusts in the Pacific Ocean and link growth episodes to fluctuating phosphorus concentrations that may be related to climate fluctuations. I am only qualified to comment on the U-Pb geochronology but I think that this may be the most unique and important contribution of the work. I found the manuscript to be generally well written and well organized with a few exceptions that are noted on the annotated manuscript. Other comments and suggestions for improvement are given below. C1-1: Lines 81-92. This paragraph lists the U-Pb dating results in text form and as such is hardly readable. These results should be listed in a table where they will be much easier for the reader to digest. The concordia plots are given in a separate Supplementary Data file that is not included in the zip file with the other supplementary files, which created some confusion on my part trying to find it, especially as the files are not labelled with their content. Ideally, all supplementary files should be accessible together. C1-2: Line 326- (Line Trace elements and U-Pb dating of CFA using LA-ICP-MS). I could not find any mention of the degree of oxidation in the plasma as measured by ThO/Th from the standard used for tuning. This should normally be set at no higher than 0.5% by regulating the nebulizer Ar flow. Raising the Ar flow will raise Pb sensitivity but cause a significant fraction of the U signal to be lost by oxidation. This is okay if the sample and standard have the same X (left-right) position in the ablation chamber. If there is any change in Y (in-out movement) helium can sometimes leak around the O ring seals that surround the exit tube (at least on some ablation chambers like EMI). This has the same effect as varying Ar flow and changes oxidation in the plasma. This will introduce significant (and uncorrectable) bias in the measured 206Pb/238U ratios unless oxidation is kept low. C1-3: The authors make no mention of whether they pre-ablated the analytical scan lines to remove surface Pb, which can be significant (U is usually not). Pre-ablations are usually carried out using a fast scan with a slightly larger bean size than that used for analysis, which here used a 160 micron beam size, the maximum for the type of laser used. It should be pointed out that even if they omitted pre-ablation and the data include significant common Pb, it should have roughly the same 207Pb/206Pb ratio as the samples since they are relatively young. This would reduce the radiogenicity of the data, hence increasing age errors, but is unlikely to result in inaccurate ages. C1-4: Which MAD standard did the authors use? There are two of them having different ages. C1-5: The authors were wise to use scans so as to avoid the uncertainty of downhole Pb/U fractionation. They seem to have integrated the U and Pb counts for each scan, as would normally be done for a spot analysis. My experience with analyzing carbonates is that many of them show significant variation in 206Pb/238U ratio along a scan, which means that results from individual measurement cycles are more spread out along the isochron, although their errors are much larger because they represent fewer counts. Regression of the three mass counts in a 3D space using the Utilchron program (https://doi.org/10.1016/j.chemgeo.2021.120454) was developed to deal with this. In the present case, it is unnecessary because the Y (207Pb/206Pb) intercept of the isochrons has been precisely fixed by analyses of Pb in the crusts. These results are consistent with model estimates for average crust such as Stacey and Kramers. Therefore extending data along the isochron is not as important as if the isochron determined both age and initial Pb composition but the authors might want to compare regression methods. C1-6: It would be helpful to label the Supplementary Data so that the reader can know what they contain before opening them. C1-7: Suppl. Table 2 (U-Pb data) There is insufficient isotopic information given to calculate error correlations so data cannot be accurately replotted and ages recalculated by the reader. Error correlations (rho values) should be given in the output of the data reduction program and it is essential that they be listed in the data table. Alternatively, giving the errors on a third ratio like 235/207 would allow rho to be calculated but it is best to list rho values themselves. Additional comments in the annotated PDF 1) CFA age data and groupings are not correctly displayed in text, which need to be displayed in table. 2) Lines 96 98 read self-evident. And there is an inappropriate use of words in this paragraph. 3) There should only be two limitations in lines 110 119 on Sr isotope dating, and the other one is not really a concern specific to Sr isotope dating. There is also some poor wording. 4) Lines 119 124, the advantages of U-Pb dating are only two. For U-Pb dating two independent systems are not clearly expressed, and for Mesozoic and younger ages, only the 206Pb/238U ratios can be practically used to obtain ages of useful precision. 5) Lines 127 129, inappropriate wording renders the sentences difficult to understand.", "reviewer_2": "Iron-manganese crusts on seamounts have proven to be treasure troves for geochemical reconstructions of ocean chemistry through time. Particularly useful for sampling mid-ocean depth chemistry, this use by Peng et al exploits all of the benefits of crust cores, including excellent geochronology and relatively pristine crystallization of carbonate fluorapatite minerals within the crustal framework. The geochemical reconstructions and the narrow age ranges of phosphatization events recorded in these crusts seem to support some changes in ocean nutrient dynamics. The analytical results of this work are outstanding and extremely well-documented. C2-1: But I found their interpretations to be a mixed bag of overly generalized interpretations of external drivers, including continental weathering (chemical for some events, and physical for others), nutrient supply changes (phosphate for some events, and eolian iron for others), and climate (cold and warm) that lack a clear consistency of driving mechanisms. Although a similar net result can certainly be driven by driven earth processes, it seems that the authors were trying to fit phosphatization events into whatever changes were occurring in the past, an approach that may lack validity. Very little attention is given to internal reorganization of phosphate reservoirs within the various ocean basins, which changed substantially throughout the Cenozoic. C2-2: One clear phosphate event which may be related to one of the identified phosphatization events is the late Miocene-early Pliocene Biogenic Bloom, a topic of significant work over the last several decades, -Filippelli 1997 Marine Geology, and Dickens and Owen Puzzlingly, this one event with the clearest documented relationship to external and internal phosphate drivers is not mentioned here at any depth. And one of the events, the ~PETM phosphatization event, references three papers that the authors state provide evidence for a chemical weathering event at this time but that none of the papers claim (indeed, they all covered timeframes outside of the PETM). C2-3: My suggestion for a revision is to frame the interpretations more cautiously rather than note a different driver for each of the observed phosphatization events, perhaps consider how internal versus external changes in phosphate supply might be reflected in the crusts, and explore a bit more the well-documented transition in the distribution of phosphate between the ocean basins as deep ocean circulation changed. I am concerned that the focus on identifying discrete drivers for each event distracts from an otherwise analytically excellent and provocative paper.", "reviewer_3": "changes from the Late Cretaceous to the present by using the ferromanganese crusts. As they have mentioned properly in the paper, the timing and environments of phosphatization are the most important issues in conjunction with ferromanganese oxide precipitation processes. It is known that the phosphate deposits occur at relatively shallow modern water depths above 1500 m such as near continental shelves, gulfs, and basins or lakes, but not often at modern deep waters. However, the modern precipitation processes are now well characterized or understood. Going back to geological ages, we know much less when, where, and how the P deposits are formed. It is generally not easy to understand the origin, environments, and chemical/mineralogical forms. C3-1: The authors should review our modern knowledge about the marine P deposits in the modern and ancient oceans, but unfortunately now well reviewed. C3-2: Their methods to approach the goal are suitable, such as fine-scale dating (U-Pb, Os, Sr isotopes), fine-scale chemical analyses (LA-ICP-MS, others), as well as growth structure analysis. Their two major points of the paper looks, 1) fine-scale and long-term age models using the comprehensive methods, and 2) environments and droving forces of phosphatization epochs. They focus on the age model of ferromanganese crust microstratigraphy in connections with phosphatization events, but they failed unfortunately to review and discuss the process and environments of ancient and modern phosphatization. They often mention the oxygen minimum environments as triggers of phosphatization. But they do not discuss where is P from, how and when is the apatite from, what is diagenesis, what is the chemical/mineralogical processes to form apatites; these are still open questions in the paper. They simply define as the stratigraphic unit without microstructural analysis, but their CFA aggregates often seem a drop stone, a fossil, vein fill, or sheets, but not like the sound evidence of events of phosphatization formation or lamella. C3-3: Prior to conducting U-Pb dating on CFA, we determined the growth interruptions and age of the main Fe-Mn crust sample MCSD85 using Os isotope chronology. C3-4: Figures 1,2,3,4 are colorful and clean, but does not simply suggest clear new ideas. C3-5: Finally, the Reviewer suggests the authors rewrite the paper, focusing only on the age model by using theU&Th decay series. Please do not use the easy empirical Co-chronology, which was used to be applied only to the hydrogenetic crusts locally in the mid-Pac seamounts. The Be-10 dating is another reliable method, and why not micropaleontology? C3-6: The authors may like to deeply discuss phosphatization, but just mention continuous or uncontinuous growth. It is because the discussion of phosphatization processes is not clearly written with sound and new findings."}, "author_response": "Response: We greatly appreciate that the reviewer acknowledges the novelty of this paper, and the valuable suggestions provided. Please see our detailed responses to each comment below, and we have also addressed the comments provided on the annotated manuscript. Please see the revised manuscript for details. [Author response continues for all reviewers...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised version of the manuscript is much improved. I only have a couple of minor modifications to suggest, which are marked at lines 68 and 69 in the annotated copy of the manuscript. Although I am not a specialist in Fe-Mg crust formation, I agree with the comments of the other two reviewers to the revised version.", "reviewer_2": "The authores successfully addressed my only concerns about the implications of this paper--namely, the complex role that internal and external variations in phosphorus delivery and internal redistribution play on the marine phosphorus cycle. The interpretations are now both more measured and more valid. I also reviewed the detailed comments for the other reviewers, and based on my reading, these were adequately addressed as well. I have no further suggestions for this excellent contribution.", "reviewer_3": "on the long-range ocean Fe- Mn crusts. In the paper they applied the two radiochemical dating methods, Os and U/Pb and presented a likely age model. I agree that this style and scope is reasonable, and partly, they had reached a nice possible solution. Nevertheless, I suggestare that the authors should accumulate more independent chronology data before they finally conclude that their method is promising. Another technical recommendation are; 1) they need careful verification of their crust samples if the one-site occurrences and sampling methods are appropriate for these delicate microstratigraphic analyses; and 2) similarly need more comprehensive microstructural, chemical and mineralogical description over the their long-range continuous growth columns without any serious secondary alteration, break, or overturns. As the reviewer strongly recommends the authors additional chronological analyses of isotopic ages by Sr and Be together with their U/Pb and Os, as the age model is a key of the paper. Regarding their idea of possible expansion of OMZ in the Cenozoic age, they can refer the Ore Geology Paper (2017) \"Formation age of the dual structure and environmental change recorded in hydrogenetic ferromanganese crusts from Northwest and Central Pacific seamounts.\" Again, the reviewer believes that the paper is worth publication in future with additional important supporting data on any specific technical journal."}, "author_response": "Response: We are grateful for the supportive comments on our manuscript and for the detailed and insightful comments, which we have found very useful in revising it. We have supplemented these parts of the manuscript in detail and respond to the comments below. [Author response continues...]"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed all of my previous concerns from the first two versions of the manuscript. I would just suggest that the caption .", "reviewer_3": "The reviewer recognized that the original manuscript was better improved, and appreciate their efforts to make the manuscript more readable and comprehensive with additional data and careful explanation with appropriat phosphatization events since the Late Cretaceous. The presented evidences in the paper are impregnation of apatite infilling and fragment of phosphatized limestone, which might lead us to quite uncertain events (or key beds) , i.e. in Supplementary Fig. 8. Another concern is their age model. Most of the calculated age data are based on sparse intervals (e.g., 6-8 mm for Os) which may easily skip important spikes. Moreover, should not be applied on all these FMC. The Sr isotope dating should be carefully reexamined (see Vonderhaar et al., 1995, GCA) which can lead us to uncertain age data. As the reviewer strongly recommends the authors additional chronological analyses of isotopic ages by Sr and Be together with their U/Pb and Os, as the age model is a key of the paper."}, "author_response": "Response: We appreciate the comments on our work, and have carefully revised the manuscript accordingly. Below is a synopsis of the main comments, and we then respond to these questions in more detail. [Author response continues...]"}, {"round": 4, "reviewer_comments": {"reviewer_4": "The authors present a very interesting paper. I have looked at the author's responses and I think on the whole they answered most of them. I still have some moderate to major concerns I explain below (especially for the U-Pb dating and EPMA). Many papers have shown that CFA may host unexpected low 207Pb/206Pb ratios (see below for references). If the authors successfully explain why they anchor the CFA 207Pb/206Pb ratios for the intercept age calculation, I believe that the paper deserves to be published because U-Pb dating is not an easy proxy when applied on CFA. The authors may have explained it in the previous rounds the text. Probably, there is a link with the crystallite size. I also really enjoyed the reading. Considering my above concern, I cannot say if the authors are right about the 6 phosphatization events."}, "author_response": "Response: We are glad that the reviewer feels our contribution is very interesting and that the paper deserves to be published. We appreciate the constructive feedback and comments, particularly the suggestions on the EPMA and U-Pb dating sections, and we have undertaken our revisions with great effort to refine our manuscript. [Author response continues...]"}, {"round": 5, "reviewer_comments": {"reviewer_4": "I have read the revised paper and the response to Referees letter. I appreciate the considerable effort made by the authors to explain the different concerns I raised previously. This is a very good paper to me with a better understanding of the U-Pb systematic on sedimentary CFA."}, "author_response": "Response: We are grateful for the detailed review of our manuscript and constructive suggestions in the last round. We are happy that our paper contributes to a more comprehensive understanding of U-Pb systematics in sedimentary CFA. We respond to the additional comments below."}]

Decision letter