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N2 fixation in low-latitude surface waters dominates the input of fixed nitrogen (N) to the global ocean, sustaining ocean fertility. In the Caribbean Sea, higher foraminifera-bound (FB-)delta 15N indicates a decline in N2 fixation during ice ages, but its cause and broader implications are unclear. Here, we report three additional Atlantic FB-delta 15N records, from the subtropical North and South Atlantic gyres (MSM58-50 and DSDP Site 516) and the equatorial Atlantic (ODP Site 662). Similar glacial and interglacial delta 15N in the equatorial Atlantic suggests a stable delta 15N for the nitrate below the gyre thermoclines. The North Atlantic record shows a FB-delta 15N rise during the ice ages, resembling a previously published FB-delta 15N record from the South China Sea. The commonality among the FB-delta 15N records is that they resemble sea level-driven variation in regional shelf area, with high FB-delta 15N (inferred reduction in N2 fixation) during periods of low shelf area. The South China Sea shows the largest delta 15N signal, the subtropical North Atlantic shows less, and the South Atlantic shows the least, the same ordering as the ice age reductions in continental shelf area in the different regions. Reduced shelf sedimentary denitrification would have increased the nitrogen-to-phosphorus ratio of the nutrient supply to open ocean surface waters, leading to decreased N2 fixation and thus higher gyre thermocline nitrate delta 15N, explaining the higher FB-delta 15N of peak ice ages. These observations identify shelf sediment denitrification as an important regional driver of modern N2 fixation and imply strong basin-scale coupling of fixed nitrogen losses and inputs. Nitrogen fixation plays the crucial role in the ocean of supplying bioavailable nitrogen (N), a major nutrient for phytoplankton growth. Variations in nitrogen fixation over time can, therefore, significantly impact ocean productivity and, consequently, carbon sequestration in the ocean int

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info:eu-repo/semantics/nonPublished

Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior, thereby controlling the distributions of many elements and their isotopes in the ocean. Through this carbon cycle feedback, changing ocean productivity has long been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. To reconstruct climate impacts from temporal changes in paleoproductivity, robust proxies are needed to test the connection between past ocean productivity, nutrient biogeochemistry, ocean circulation and climate. Here we compile water column carbon (C), nitrogen (N) and silicon (Si) stable isotopes from GEOTRACES-era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient partitioning based on the C, N, and Si isotopic composition of marine sediments. Relationships between water column isotope distributions, ocean productivity, and nutrient utilization are discussed. The potential for isotope measurements in sedimentary archives to record aspects of past ocean productivity are evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on past ocean productivity, nutrient cycling and utilization during late Quaternary glacial-interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement proxy applications and emphasizes the importance of such applications to reconstructing Cenozoic climate evolution.

The Southern Ocean is widely recognized as a potential cause of the lower atmospheric concentration of CO2 during ice ages, but the mechanism is debated. Focusing on the Southern Ocean surface, we review biogeochemical paleoproxy data and carbon cycle concepts that together favor the view that both the Antarctic and Subantarctic Zones (AZ and SAZ) of the Southern Ocean played roles in lowering ice age CO2 levels. In the SAZ, the data indicate dust-driven iron fertilization of phytoplankton growth during peak ice age conditions. In the ice age AZ, the area-normalized exchange of water between the surface and subsurface appears to have been reduced, a state that we summarize as “isolation” of the AZ surface. Under most scenarios, this change would have stemmed the leak of biologically stored CO2 that occurs in the AZ today. SAZ iron fertilization during the last ice age fits with our understanding of ocean processes as gleaned from modern field studies and experiments; indeed, this hypothesis was proposed prior to evidentiary support. In contrast, AZ surface isolation is neither intuitive nor spontaneously generated in climate model simulations of the last ice age. In a more prospective component of this review, the suggested causes for AZ surface isolation are considered in light of the subarctic North Pacific (SNP), where the paleoproxies of productivity and nutrient consumption indicate similar upper ocean biogeochemical changes over glacial cycles, although with different timings at deglaciation. Among the proposed initiators of glacial AZ surface isolation, a single mechanism is sought that can explain the changes in both the AZ and the SNP. The analysis favors a weakening and/or equatorward shift in the upwelling associated with the westerly winds, occurring in both hemispheres. This view is controversial, especially for the SNP, where there is evidence of enhanced upper water column ventilation during the last ice age. We offer an interpretation that may explai, SCOPUS: re.j, info:eu-repo/semantics/published

We report the differences between δ15N of organic nitrogen bound in foraminifera tests (foraminifera‐bound δ15N, or FB‐δ15N), which is considered an effective recorder of thermocline nitrate δ15N in the oligotrophic areas, and bulk sedimentary δ15N (δ15Nbulk) of core MD12‐3433, located in the northern South China Sea (SCS), to explore possible causes of the offset between these two proxies in the SCS. The offset can be best explained by the influence of the terrigenous input on δ15Nbulk, as evidenced by its general correlation with sea level changes, C:N ratios, and inorganic nitrogen contribution to total nitrogen. Moreover, our new record is compared to six previously published δ15Nbulk records of the SCS based on their geographical proximity to the mainland or large rivers, which all show being influenced by terrigenous input. The effect of changes in vertical mixing is evaluated and believed to play a minor role on the general pattern of FB‐δ15N, while N2 fixation is the most important impact on FB‐δ15N records in the SCS as previously studies revealed. A deeper mixed layer during the last glacial period could result in nitrate with higher δ15N from the subsurface being mixed into the euphotic zone. FB‐δ15N of Orbulina universa have higher values than Globigerinoides ruber and Globigerinoides sacculifer, with a maximum difference in the last glacial maximum, which could be accounted for by its deeper habitat where it is sensitive to changes in the contributions of vertically mixed subsurface nitrate relative to nitrate from N2 fixation.

Cold, nutrient-and CO2-rich waters upwelling in the eastern equatorial Pacific (EEP) give rise to the Pacific cold tongue. Quasi-periodic subsidence of the thermocline and attenuation in wind strength expressed by El Niño conditions decrease upwelling rates, increase surface-water temperatures in the EEP, and lead to changes in regional climates both near and far from the equatorial Pacific. EEP surface waters have elevated CO2concentrations during neutral (upwelling) or La Niña (strong upwelling) conditions. In contrast, approximate air–sea CO2equilibrium characterizes El Niño events. One hypothesis proposes that changes in physical oceanography led to the establishment of a deep tropical thermocline and expanded mixed-layer prior to 3 million years ago. These effects are argued to have substantially reduced deep-water upwelling rates in the EEP and promoted a “permanent El Niño-like” climate state. For this study, we test this supposition by reconstructing EEP “excess CO2” and upwelling history for the past 6.5million years using the alkenone-pCO2methodology. Contrary to previous assertions, our results indicate that average temporal conditions in the EEP over the past ∼6.5 million years were characterized by substantial CO2disequilibrium and high nutrient delivery to surface waters — characteristics that imply strong upwelling of deep waters. Upwelling appears most vigorous between ∼6.5 to 4.5 million years ago coinciding with high accumulation rates of biogenic material during the late Miocene – early Pliocene “biogenic bloom”.