Paleoceanographic Proxies Research Articles

Worldwide consequences of a mid-Holocene cold event in the Nordic Seas

The present interglacial is a relatively warm and stable period, especially compared to the preceding glacial time. However, the Holocene has seen the emergence of several remarkable cold events, some with worldwide consequences. Leveraging marine records from the Nordic Seas, we provide the first detailed account of a cold event centered around 6.8 ka BP. Utilizing paleoceanographic proxies and advanced modeling, we unveil a distinct subsurface water cooling, associated with a stepwise increase in sea-ice cover in the eastern Fram Strait. Our findings emphasize the role of Greenland Sea deep convection onset and the subsequent westward shift in Atlantic Water flow, enabling sea-ice advection from the Barents Sea. The heightened sea-ice cover weakened Atlantic Water advection, perturbing thermohaline circulation in the eastern Nordic Seas. These perturbations propagated worldwide, affecting North Atlantic deep-water circulation, inducing widespread hemispheric cooling, shifting the Intertropical Convergence Zone southward, and weakening the East Asian monsoon. Incorporating results from the Transient simulations of Climate Evolution of the last 21,000 years (TraCE-21ka) supports and augments proxy-based paleoreconstructions, underscoring sea-ice dynamics and ocean circulation's critical influence. This study highlights the potential for localized cold events within ostensibly warm climatic intervals. It underscores the need to comprehend their mechanisms for precise climate predictions and informed policymaking toward a sustainable future.

Occurrence characteristics of authigenic pyrite in the deep-sea environment and its paleoceanographic implications based on core sediments from IODP Expedition 342 Site U1406

Although authigenic pyrite (FeS2) in marine sediment is an important proxy for oxygen conditions and microbial activities at its formation site, very little is known about its morphological characteristics in the deep-sea (> 3500 m) environment and their paleoceanographic implications. Here, we report the occurrence characteristics of authigenic pyrite including texture, size distribution, and relative content in North Atlantic deep-sea sediments from Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406. The results of electron microscopy show octahedral pyrite microcrystals of 1.5–4.0 μm, the spherical texture of greigite (Fe3S4), and large pyrite framboids (average, 25 μm) relative to those of the continental margin. These features may demonstrate an unchanged supersaturation level of the pore water for octahedral pyrite after its initial formation, replacement of the precursor greigite by pyrite, and an expanded pyrite framboid formation zone with a low sulfate reduction rate in deep-sea sediments, respectively. Although the size distribution of pyrite framboids measured in this study falls into the range of pyrites formed in the sulfate–methane transition zone, based on onboard geochemical data and previous research, these pyrites most likely formed through organoclastic sulfate reduction during diagenesis. We assessed the implications of these findings for paleoceanographic changes, as the formation of diagenetic pyrite is controlled by the supply rate and reactivity of organic matter, which are closely related to ocean productivity and circulation. A comparison between the profiles of relative pyrite content and benthic foraminiferal oxygen isotope (δ18Obf) values indicated that these factors were anti-phased during the late Oligocene. Based on our results, we conclude that the formation of diagenetic pyrite was favored during interglacial periods due to increases in the amount and reactivity of organic matter driven by more energetic formation of proto-North Atlantic Deep Water. Our findings provide new insights into the formation of authigenic pyrite in deep-sea environments and its potential as a paleoceanographic proxy.

Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Inorganic (Carbonate) Carbon Cycle and Feedbacks

AbstractAstronomical cycles are strongly expressed in marine geological records, providing important insights into Earth system dynamics and an invaluable means of constructing age models. However, how various astronomical periods are filtered by the Earth system and the mechanisms by which carbon reservoirs and climate components respond, particularly in absence of dynamic ice sheets, is unclear. Using an Earth system model that includes feedbacks between climate, ocean circulation, and inorganic (carbonate) carbon cycling relevant to geological timescales, we systematically explore the impact of astronomically‐modulated insolation forcing and its expression in model variables most comparable to key paleoceanographic proxies (temperature, the δ13C of inorganic carbon, and sedimentary carbonate content). Temperature predominately responds to obliquity and is little influenced by the modeled carbon cycle feedbacks. In contrast, the cycling of nutrients and carbon in the ocean generates significant precession power in atmospheric CO2, benthic ocean δ13C, and sedimentary wt% CaCO3, while inclusion of marine sedimentary and weathering processes shifts power to the long eccentricity period. Our simulations produce reduced pCO2 and dissolved inorganic carbon δ13C at long eccentricity maxima and, contrary to early Cenozoic marine records, CaCO3 preservation in the model is enhanced during eccentricity modulated warmth. Additionally, the magnitude of δ13C variability simulated in our model underestimates marine proxy records. These model‐data discrepancies hint at the possibility that the Paleogene silicate weathering feedback was weaker than modeled here and that additional organic carbon cycle feedbacks are necessary to explain the full response of the Earth system to astronomical forcing.

Differential dissolution of biogenic silica significantly affects the utility of sediment diatoms as paleoceanographic proxies

AbstractDiatoms and biogenic silica (biogenic Si) in sediments are commonly analyzed as paleoceanographic environmental indicators. However, the correspondence between these sedimentary components and their counterparts in the water column above can vary over time and space. This study, undertaken in the northern South China Sea, compares diatoms and biogenic Si in sinking particles from the water column with those in surface sediments below. The results indicate substantial losses of diatoms and biogenic Si during sinking and burial, along with changes in diatom assemblages. About 6% of diatoms sinking from 1000 m depth are estimated to be preserved in sediments: 32% are recycled in the water column and 62% are lost at the sediment–water interface. Approximately, 12% of biogenic Si is preserved in sediments: 6% is remineralized in the water column and 82% is lost at the sediment–water interface. The differences between diatom and biogenic Si preservation and remineralization are likely due to differences in dissolution resistance among the various diatom species and between diatoms and other silicifiers (e.g., radiolarians), along with effects of lateral transport. This variability introduces uncertainties into paleoceanographic inferences made from these two proxies from the sediments. Thalassionema nitzschioides was found to dominate sediment assemblages, which we attribute to its great resistance to dissolution, and to be negatively correlated with diatom sinking fluxes. These properties suggest that in the northern South China Sea, T. nitzschioides may potentially be useful as an indicator of low paleoproductivity (e.g., as seen during strong El Niño events).

Late Cenomanian Plenus event in the Western Interior Seaway

The Late Cenomanian Plenus Cold Event is one of the most enigmatic paleoclimate episodes in Earth history with potential to inform understanding of global climate system variability under greenhouse warming conditions, as well as internal feedback pathways that modulate such variability. Following an interpreted massive addition of volcanic CO2 to the atmosphere and warming that led to a major ocean anoxic event (OAE2), there was a brief interval of cooling recorded in oxygen isotopes and biogeographic data. Here we present evidence that cooling was absent or muted within the Western Interior Seaway (WIS). Clumped isotope data from the basin suggest persistent extreme warmth during the Late Cenomanian, macroinvertebrate fossil assemblages do not record a decrease in temperature, and changes in other paleoceanographic proxies do not correlate temporally with Plenus interval signals from other locales. Using select proxy data to guide construction of GCM model simulations, we explore possible hypotheses to explain these observations. Our results suggest that the paleogeographic configuration of the basin and its gateways to adjoining oceans, which evolved in association with changing pCO2 and sea level, influenced winter sea ice formation at the northern aperture of the seaway, water mass circulation, salinity, temperature, and water column stratification. We propose that northward advection of warm Tethyan water muted expression of Plenus cooling in the seaway. Understanding the unique character of the Western Interior paleoceanographic record provides critical input for the development of robust models of ancient Earth System dynamics and should aid predictions of future climate system dynamics.

A synthesis of Cenozoic paleoceanographic proxies for seawater oxygenation

A synthesis of Cenozoic paleoceanographic proxies for seawater oxygenation

Anomalous 234U/238U isotopic composition in Southern Ocean sediments

Uranium is a redox-sensitive trace element that under certain sedimentary conditions will reflect changes in past biological productivity and/or deep ocean ventilation. Applications for the isotopic composition of U (234U:238U activity ratio, δ234U or the per mil deviation from secular equilibrium) as a paleoceanographic proxy, however, remain largely unexplored in pelagic sediments. We present δ234U of bulk U for a 504-kyr record of South Atlantic core ODP 1094, a record that has exhibited sensitivity to bottom water formation with minimal 238U remobilization due to changes in redox condition. Sedimentary δ234U at time of burial was modeled given authigenic and detrital sources, where it was found that δ234U has a reoccurring pattern of anomalous behavior around deglacial transitions. In many cases, the anomalous δ234U compositions in sediment exceeded seawater, which is not consistent with its expected sources. A mass balance of 234U across these anomalous sections showed that vertical rearrangement of 234U can explain anomalous sedimentary δ234U during Marine Isotope Stage (MIS) 7/8 and 11/12 transitions, as well as partially during MIS 5/6 and MIS 9/10. The distribution of these anomalies is likely consistent with the idea that deglacial spikes in aU, resulting from periods of high organic carbon delivery and thereby strong reduction in the sediments, act as a persistent sink of 234U alpha-recoiled from vertically adjacent sediment layers after burial. While this mechanism explains the negative δ234U anomalies, there is a large subset of positive δ234U anomalies that cannot be explained by vertical diffusion of 234U in pore waters alone. This excess 234U points to an extremely high δ234U source to seawater during deglaciation or additional diagenetic effects such as diffusion from lateral heterogeneity in the redox state of sediment at depth. We find that δ234U of pelagic sediment may be useful to identify fine-scale pore water migration of alpha-recoil products.

Decoupling of Barium and Silicon at the Congo River‐Dominated Southeast Atlantic Margin: Insights From Combined Barium and Silicon Isotopes

AbstractThe correlation between concentrations of dissolved barium (dBa) and silicon (dSi) in the modern ocean supports the use of Ba as a paleoceanographic proxy. However, the mechanisms behind their linkage and the exact processes controlling oceanic Ba cycling remain enigmatic. To discern the extent to which this association arises from biogeochemical processes versus physical mixing, we examine the behavior of Ba and Si at the Congo River‐dominated Southeast Atlantic margin where active biological processes and large boundary inputs override the large‐scale ocean circulation. Here we present the first combined measurements of dissolved stable Ba (δ138Ba) and Si (δ30Si) isotopes as well as Ba and Si fluxes estimated based on 228Ra from the Congo River mouth to the northern Angola Basin. In the surface waters, river‐borne particle desorption or dissolution and shelf inputs lead to non‐conservative additions of both dBa and dSi to the Congo‐shelf‐zone, with the Ba flux increasing more strongly than that of Si across the shelf. In the epipelagic and mesopelagic layers, Ba and Si are decoupled likely due to different depths of in situ barite precipitation and biogenic silica production. In the deep waters of the northern Angola Basin, we observe large enrichment of dBa, likely originating from high benthic inputs from the Congo deep‐sea fan sediments. Our results reveal different mechanisms controlling the biogeochemical cycling of Ba and Si and highlight a strong margin influence on marine Ba cycling. Their close association across the global ocean must therefore mainly be a consequence of the large‐scale ocean circulation.

Distribution of stable isotopes of Mo and W from a river to the ocean: signatures of anthropogenic pollution

Molybdenum and tungsten are redox-sensitive elements, and their stable isotope ratios have attracted attention as paleoceanographic proxies. However, our knowledge of the distribution of stable Mo and W isotopes in the modern hydrosphere remains limited. In this study, we provided the concentrations and isotope ratios of dissolved Mo and W in the oceans (the North Pacific and Indian Oceans), marginal seas (the East China Sea and Sea of Japan), and a river-estuary system in Japan (from the Uji-Yodo rivers to Osaka Bay). In the North Pacific and Indian Oceans, the W concentration was 48.2 ± 6.2 pmol/kg (ave ± 2sd, n = 109), δ186/184W was 0.52 ± 0.06 ‰, the Mo concentration was 105.1 ± 8.0 nmol/kg, and δ98/95Mo was 2.40 ± 0.06 ‰. The results indicate that W has the constant concentration and isotopic composition in the modern ocean as well as Mo. In the East China Sea and the Sea of Japan, the W concentration and δ186/184W in the upper water (< 1000 m depth) were different from those in the ocean (W = 56 ± 18 pmol/kg, δ186/184W = 0.45 ± 0.06 ‰, n = 24). However, the concentrations in deeper water were congruent with those in the oceans (W = 49.9 ± 7.6 pmol/kg, δ186/184W = 0.50 ± 0.02 ‰, n = 7). The Mo concentration was 105.4 ± 3.1 nmol/kg and δ98/95Mo was 2.36 ± 0.03 ‰ (n = 31) throughout the water column, congruent with those in the ocean. In the Uji River-Yodo River-Osaka Bay system, the W concentration reached 1074 pmol/kg and δ186/184W reached 0.20 ‰. We propose that the enrichment of W with a low δ186/184W in the river-estuary system and marginal seas is caused by anthropogenic pollution. Anthropogenic Mo pollution was not detected in marginal seas. However, the Mo concentration and δ98/95Mo showed high anomalies above the mixing line of river water and seawater in the lower Yodo River and Osaka Bay, implying possible anthropogenic pollution of Mo in the metropolitan area.

Biomarker characterization of the North Water Polynya, Baffin Bay: implications for local sea ice and temperature proxies

Abstract. The North Water Polynya (NOW, Inuktitut: Sarvarjuaq; Kalaallisut: Pikialasorsuaq), Baffin Bay, is the largest polynya and one of the most productive regions in the Arctic. This area of thin to absent sea ice is a critical moisture source for local ice sheet sustenance and, coupled with the inflow of nutrient-rich Arctic Surface Water, supports a diverse community of Arctic fauna and indigenous people. Although paleoceanographic records provide important insight into the NOW's past behavior, it is critical that we better understand the modern functionality of paleoceanographic proxies. In this study, we analyzed lipid biomarkers, including algal highly branched isoprenoids and sterols for sea ice extent and pelagic productivity and archaeal glycerol dibiphytanyl glycerol tetraethers (GDGTs) for ocean temperature, in a set of modern surface sediment samples from within and around the NOW. In conjunction with previously published datasets, our results show that all highly branched isoprenoids exhibit strong correlations with each other and not with sterols, which suggests a spring or autumn sea ice diatom source for all highly branched isoprenoids (HBIs) rather than a combination of sea ice and open-water diatoms as seen elsewhere in the Arctic. Sterols are also highly concentrated in the NOW and exhibit statistically higher concentrations here compared to sites south of the NOW, consistent with the order of magnitude higher primary productivity observed within the NOW relative to surrounding waters in spring and summer months. Finally, our local temperature calibrations for GDGTs and OH-GDGTs reduce the uncertainty present in global temperature calibrations but also identify some additional variables that may be important in controlling their local distribution, such as nitrate availability and dissolved oxygen. Collectively, our analyses provide new insight into the utility of these lipid biomarker proxies in high-latitude settings and will help provide a refined perspective on the past development of the NOW with their application in downcore reconstructions.

Next-generation sequencing and metabarcoding to understand the ecology of benthic foraminiferal community in the Bering Sea

The sedimentary archives from the Bering Sea can provide excellent records of the global climate shift. However, paleoclimatic studies from the Bering Sea are limited due to the lack of well calibrated proxies. Various characteristics of foraminifera are frequently used as paleoclimatic and paleoceanographic proxies. A precise understanding of the ecology of foraminiferal species is required to use it as proxy. Here, we use next-generation sequencing and metabarcoding to understand the ecology of benthic foraminiferal community in the Bering Sea. The total genomic deoxyribonucleic acid (DNA) was extracted from 11 surface sediment samples collected from the Bering Sea. The foraminiferal small sub unit (SSU) rDNA was amplified with foraminiferal-specific primers. A total of 2,170,762 effective tags and 1046 operational taxonomic unit (OTUs) were obtained after next-generation sequencing, strict quality control, and data processing. The results showed that the station BR07 had the highest number of OTUs. The highest foraminiferal Margalef index and Shannon-Wiener index were obtained at BR06. Rotaliida accounted for the highest proportion, followed by Textulariida and Monothalamida, with the lowest proportion of Miliolida. From the principal component analysis (PCA), Leptohalysis scottii, Stainforthiidae_X sp., and Rectuvigerina phlegeri were found to be the more important species in the Bering Sea. Based on the redundancy analysis (RDA) and Spearman correlation analysis, we found that the grain size and water depth exhibited significant control on foraminiferal community. The foraminiferal reads were positively correlated with the grain size, whereas the Shannon-Wiener index was negatively correlated with it. The water depth was positively correlated with OTUs, Leptohalysis scottii, and Rectuvigerina phlegeri. The clustering analysis showed that Bering Sea stations cluster together with Yellow Sea stations of similar water depth. The water depth was a bigger factor for foraminiferal community than the geographical distance.

Increased terrigenous input from North America to the northern Mendeleev Ridge (western Arctic Ocean) since the mid-Brunhes Event

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene.

Foraminifera as a model of the extensive variability in genome dynamics among eukaryotes.

Knowledge of eukaryotic life cycles and associated genome dynamics stems largely from research on animals, plants, and a small number of "model" (i.e., easily cultivable) lineages. This skewed sampling results in an underappreciation of the variability among the many microeukaryotic lineages, which represent the bulk of eukaryotic biodiversity. The range of complex nuclear transformations that exists within lineages of microbial eukaryotes challenges the textbook understanding of genome and nuclear cycles. Here, we look in-depth at Foraminifera, an ancient (∼600million-year-old) lineage widely studied as proxies in paleoceanography and environmental biomonitoring. We demonstrate that Foraminifera challenge the "rules" of life cycles developed largely from studies of plants and animals. To this end, we synthesize data on foraminiferal life cycles, focusing on extensive endoreplication within individuals (i.e., single cells), the unusual nuclear process called Zerfall, and the separation of germline and somatic function into distinct nuclei (i.e., heterokaryosis). These processes highlight complexities within lineages and expand our understanding of the dynamics of eukaryotic genomes.

Global Quaternary Carbonate Burial: Proxy- and Model-Based Reconstructions and Persisting Uncertainties.

Constraining rates of marine carbonate burial through geologic time is critical for interpreting reconstructed changes in ocean chemistry and understanding feedbacks and interactions between Earth's carbon cycle and climate. The Quaternary Period (the past 2.6 million years) is of particular interest due to dramatic variations in sea level that periodically exposed and flooded areas of carbonate accumulation on the continental shelf, likely impacting the global carbonate budget and atmospheric carbon dioxide. These important effects remain poorly quantified. Here, we summarize the importance of carbonate burial in the ocean-climate system, review methods for quantifying carbonate burial across depositional environments, discuss advances in reconstructing Quaternary carbonate burial over the past three decades, and identify gaps and challenges in reconciling the existing records. Emerging paleoceanographic proxies such as the stable strontium and calcium isotope systems, as well as innovative modeling approaches, are highlighted as new opportunities to produce continuous records of global carbonate burial.

Photosynthetic Carbon Assimilation and Electron Transport Rates in Two Symbiont-Bearing Planktonic Foraminifera

Photosymbiosis is one of the key features characterizing planktonic foraminifera; the number of symbiont cells within a single host has been reported to be well over thousands, meaning that photosynthesis by photosymbiosis may be a “hot spot” for primary production, especially in oligotrophic oceans. As microenvironmental conditions around foraminifera are greatly affected by rapid biological activities—such as photosynthesis and respiration—information on the photosynthetic activities of symbionts is essential to interpret the geochemical proxies recorded in foraminiferal tests (e.g., δ13C and δ18O). Recently, active chlorophyll fluorometry has been increasingly employed as a useful tool for immediate estimation of photosynthesis. However, carbon assimilation rates are the only direct indicator of the photosynthetic carbon flux. Therefore, before utilizing active fluorescence methods to understand carbon dynamics in foraminiferal symbiosis, it is necessary to confirm the relationship between the fluorescence-based photosynthetic rate [electron transport rate (ETR)] and carbon assimilation rate (P). Here, these two rates were compared for two species, Trilobatus sacculifer and Globigerinella siphonifera Type II, using 14C-tracer experiments and active fluorometric measurements by fast repetition rate fluorometry. The results showed a significant positive correlation between the P and ETR of the two species, indicating that carbon assimilation can be estimated by the fluorometric method. However, the regression slopes, which represent the apparent electron requirement for carbon assimilation (e–/C), were significantly different in the two species, and were estimated at 26.2 for T. sacculifer and 96.5 for G. siphonifera. These are strikingly high, considering the theoretically and empirically realistic e–/C values. We hypothesized that the high e–/C observed may be due in part to the use of unlabeled respiratory carbon (underestimation of P). A simple mass balance calculation suggests that a significant amount of carbon should derive from the host’s respired CO2, whose contribution is higher in G. siphonifera than in T. sacculifer. Within the context of using test geochemical parameters, such as δ13C, as paleoceanographic proxies, it is important to note that the potential magnitude of the photosynthetic effect varies among species. This attempt to couple ETR and P could comprehensively reveal an interesting perspective on the intimate interactions existing within photosymbiotic systems.

Linkages Between East China Sea Deep‐Sea Oxygenation and Variability in the East Asian Summer Monsoon and Kuroshio Current Over the Last 400,000 years

AbstractThe East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic matter influx, which is controlled by the East Asian Summer Monsoon (EASM), and the increased productivity driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ13Cpf, and δ18Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid‐MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally‐paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.

Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Inorganic (Carbonate) Carbon Cycle and Feedbacks.

Astronomical cycles are strongly expressed in marine geological records, providing important insights into Earth system dynamics and an invaluable means of constructing age models. However, how various astronomical periods are filtered by the Earth system and the mechanisms by which carbon reservoirs and climate components respond, particularly in absence of dynamic ice sheets, is unclear. Using an Earth system model that includes feedbacks between climate, ocean circulation, and inorganic (carbonate) carbon cycling relevant to geological timescales, we systematically explore the impact of astronomically modulated insolation forcing and its expression in model variables most comparable to key paleoceanographic proxies (temperature, the δ13C of inorganic carbon, and sedimentary carbonate content). Temperature predominately responds to short and long eccentricity and is little influenced by the modeled carbon cycle feedbacks. In contrast, the cycling of nutrients and carbon in the ocean generates significant precession power in atmospheric CO2, benthic ocean δ13C, and sedimentary wt% CaCO3, while inclusion of marine sedimentary and weathering processes shifts power to the long eccentricity period. Our simulations produce reduced pCO2 and dissolved inorganic carbon (DIC) δ13C at long eccentricity maxima and, contrary to early Cenozoic marine records, CaCO3 preservation in the model is enhanced during eccentricity‐modulated warmth. Additionally, the magnitude of δ13C variability simulated in our model underestimates marine proxy records. These model‐data discrepancies hint at the possibility that the Paleogene silicate weathering feedback was weaker than modeled here and that additional organic carbon cycle feedbacks are necessary to explain the full response of the Earth system to astronomical forcing.

Release from biogenic particles, benthic fluxes, and deep water circulation control Cr and δ53Cr distributions in the ocean interior

Chromium (Cr) has shown promise as a paleoceanographic proxy due to the redox-driven control of dissolved Cr concentrations ([Cr]) and stable isotope composition (δ53Cr). To improve the mechanistic understanding of Cr cycling in the modern ocean and strengthen its potential proxy applications, we present new data from regeneration incubations, bottom and sediment pore waters, and a compilation of intermediate and deep water data. While Cr removal and biological export from the surface ocean is associated with organic carbon export, the deep water release of dissolved Cr from sinking particles is not directly dependent on organic carbon respiration, as indicated by differing trends between Cr, oxygen utilization and the regeneration of organic-associated macronutrients (e.g. N, P). Pore water and bottom water data demonstrate that benthic Cr fluxes are locally important and may be significant globally. The pore water dissolved Cr flux at our CaCO3-rich site is likely driven by the re-release of Cr scavenged from the water column by sinking particles, with minor contributions from lithogenic phases. We argue this is consistent with the highest open ocean [Cr] to date being found in the water column below oxygen minimum zones, likely reflecting the release of scavenged Cr in deep waters or surface sediments. Chromium released from suspended particles and surface sediments follows the global δ53Cr–[Cr] array, supporting the proposed role of biological export and regeneration in shaping global Cr and δ53Cr distributions. Global intermediate and deep water [Cr], δ53Cr and Cr:macronutrient relationships are thus shaped by a synergy of circulation patterns, water mass mixing, a deep Cr regeneration cycle, and benthic Cr sources. A biogenic control on global Cr distributions indicates that sedimentary Cr records may reflect biogenic as well as O2-dependent processes, while more research is needed to assess sediment Cr record fidelity based on an active diagenetic cycle.

Proxies for paleo-oxygenation: A downcore comparison between benthic foraminiferal surface porosity and I/Ca

Benthic foraminiferal surface porosity (the mean percentage of surface area covered by pores; higher porosity: lower oxygenation) and iodine to calcium ratio (I/Ca, higher I/Ca: higher oxygenation) are both promising paleoceanographic proxies that will advance through testing in down-core studies. Here we report the first down-core comparison (~45 kyr) of these proxies for a core from the southern Brazilian margin (26°40.22′ S, 46°26.46′ W, 475 m water depth). Both proxies are most sensitive to low-O2 conditions (<50 μmol/kg), and not well-constrained at higher O2 concentrations. Porosity values are generally low (<15%) and I/Ca ranges between ~4 and ~ 6 μmol/mol throughout the core. The two proxies are overall consistent, suggesting that bottom-water oxygen concentrations at the site remained above 50 μmol/kg during the last 45 kyr. Several non-O2 factors (e.g., iodate reduction rates, water mass mixing, temperature, foraminiferal shell robustness) could influence the proxies and require further investigation.

Impact of methane seepage dynamics on the abundance of benthic foraminifera in gas hydrate bearing sediments: New insights from the South China Sea

The stable isotopic composition of benthic foraminifera tests is widely used to trace past methane release events. However, gaps in our knowledge on the ecology of benthic foraminifera at methane seeps impede the utility of test composition as proxy of methane emission histories and the influence of redox zonation on the local fauna. Here, we use sediment geochemistry and foraminiferal assemblages of a seep area of the South China Sea to reveal that seepage duration and intensity affect benthic foraminifera. Taxa tolerating eutrophic conditions like Uvigerina thrive at times of reduced seepage when aerobic methanotrophy is favored. In contrast, intense methane seepage leads to high sulfide levels, and a positioning of the sulfate-methane transition zone close to the seafloor. At these times, bottom waters may become oxygen depleted and benthic foraminifera will decline. This study provides (1) the means to understand how seepage dynamics control the distribution of benthic foraminifera and (2) new constraints on the use of foraminifera as paleoceanographic proxies, including possible mechanisms of mass mortality in the geological past.