The marine nitrogen (N) cycle plays a central role in regulating ocean productivity, ecosystem structure, and biogeochemical feedbacks on climate. However, our understanding of historical changes in the marine N cycle is limited by the short time span and spotty spatial coverage of direct observations. This study explores the use of crustose coralline algae (CCA) as a novel proxy for reconstructing past changes in the marine N cycle. We use the biomineral-bound nitrogen isotopic composition of tropical CCA (δ15NCCA) and compare it to the δ¹⁵N of subsurface nitrate (δ15N Nitrate) and corals (δ15Ncoral). Our results show that CCA reliably track subsurface nitrate δ¹⁵N and, therefore, can be used to reconstruct past changes in the N cycle. Additionally, anchoring δ15N coral values to co-occurring CCA allows us to isolate the reliance on photosymbionts in coral species, revealing taxonomic and regional variation in trophic strategies. Therefore, δ15N CCA also provides a tool to investigate coral trophodynamics in the modern ocean as well as in the historical record.

The ability of the deep ocean to store and exchange large quantities of CO2 with the atmosphere on relatively short timescales means that it is thought to play a key role in dictating glacial-interglacial changes in atmospheric CO2, however records of deep ocean carbon storage and release remain sparse. The Pacific Ocean contains the largest store of carbon in the ocean-atmosphere system. As a result, changes in its circulation dynamics and biogeochemistry have the potential to significantly impact global climate. Despite this, changes in Pacific conditions and carbon storage over the last glacial cycle are poorly constrained.Here we present new geochemical proxy records from abyssal, deep, and intermediate depths in the Southwestern Pacific to determine the changes in deep ocean carbon storage over the last glacial cycle and the mechanisms involved in driving these changes. Foraminiferal trace element and stable isotope data indicate that increased carbon storage occurred over the course of the last glaciation, promoting a drawdown in atmospheric CO2. The processes involved in driving glacial ocean carbon storage are debated, however proxy data from these sites indicate that changes in circulation dynamics promoting the isolation and expansion of deep Pacific waters was likely a key process involved. Comparison of δ13C data to box model and Earth system model output provides further insight into the physical as well as biogeochemical mechanisms involved and their relative contributions at different stages over the last glacial cycle. This includes the role of Southern Ocean sea-ice expansion, reduced ocean temperatures, and increased Southern Ocean stratification and biological productivity. We find that physical processes dominate the early in the glacial cycle, with biological processes promoting further drawdown as glacial conditions intensify. These results help to improve the understanding of deep ocean carbon cycling over the last glacial cycle and provide a new framework with which to interpret proxy δ13C data.

Seabirds transfer nutrients from the ocean to their nesting island, potentially altering nitrogen (N) cycling within adjacent terrestrial and marine ecosystems. Yet, the processes involved in seabird-N transfer along the land-sea continuum remain elusive. Using delta 15N and delta 18O measurements of groundwater nitrate, we demonstrate the role of brackish groundwater located within a coral island's landmass as a major reservoir of nitrate (at millimolar levels). Nearly all of the total dissolved seabird-derived N leaching into the groundwater (mostly ammonium and uric acid) is converted to nitrate by nitrification, as supported by the relatively low delta 18O of the groundwater nitrate (3.97 parts per thousand +/- 0.30 parts per thousand). Comparison of nitrate delta 15N and delta 18O suggests that little denitrification takes place within the groundwater lens, implying that the high delta 15N of groundwater nitrate (13.73 parts per thousand +/- 0.05 parts per thousand) derives from the high trophic position of seabirds and postdepositional processes that increase the delta 15N of seabird excreta. Seawater and coral skeleton samples from a reef flat exposed to groundwater had higher delta 15N values than at sites devoid of groundwater influence, indicating that the main source of N at the latter site was the Subtropical Upper Water, while the groundwater nitrate dominated the exposed reef flat N pool up to 200 m from shore. In addition, these results indicate that coral-bound delta 15N can detect seabird-derived nitrate delta 15N, raising opportunities to reconstruct historical seabird-N inputs to coral reefs in relation to climatic and other changes, such as the introduction of invasive species.

AbstractSeawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial‐interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15N/14N ratio (δ15N) of mixed‐layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ15N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N2 fixation that occurs in the Indian Ocean. Foraminifera δ15N may have been lowered further during eddy migration by in situ N2 fixation and/or recycling of low‐δ15N ammonium. The absence of low‐δ15N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ15N. The low δ15N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer‐bound δ15N from the region could record variations in past Agulhas leakage.

Elevated sea surface temperatures are causing an increase in coral bleaching events worldwide, and represent an existential threat to coral reefs. Early studies of Mesophotic Coral Ecosystems (MCEs) highlighted their potential as thermal refuges for shallow-water coral species in the face of predicted 21(st) century warming. However, recent genetic evidence implies that limited ecological connectivity between shallow- and deep-water coral communities inhibits their effectiveness as refugia; instead MCEs host distinct endemic communities that are ecologically significant in and of themselves. In either scenario, understanding the response of MCEs to climate change is critical given their ecological significance and widespread global distribution. Such an understanding has so far eluded the community, however, because of the challenges associated with long-term field monitoring, the stochastic nature of climatic events that drive bleaching, and the paucity of deep-water observations. Here we document the first observed cold-water bleaching of a mesophotic coral reef at Clipperton Atoll, a remote Eastern Tropical Pacific (ETP) atoll with high coral cover and a well-developed MCE. The severe bleaching (>70 % partially or fully bleached coral cover at 32 m depth) was driven by an anomalously shallow thermocline, and highlights a significant and previously unreported challenge for MCEs. Prompted by these observations, we compiled published cold-water bleaching events for the ETP, and demonstrate that the timing of past cold-water bleaching events in the ETP coincides with decadal oscillations in mean zonal wind strength and thermocline depth. The latter observation suggests any future intensification of easterly winds in the Pacific could be a significant concern for its MCEs. Our observations, in combination with recent reports of warm-water bleaching of Red Sea and Indian Ocean MCEs, highlight that 21(st) century MCEs in the Eastern Pacific face a two-pronged challenge: warm-water bleaching from above, and cold-water bleaching from below.

The ability of scleractinian corals to thrive in the oligotrophic (low-nutrient, low-productivity) surface waters of the tropical ocean is commonly attributed to their symbiotic relationship with photosynthetic dinoflagellates 1,2 . The evolutionary history of this symbiosis might clarify its organismal and environmental roles. It is currently unclear whether photosymbiosis first arose in the Triassic, with the emergence of scleractinian corals, or if it was already prevalent amongst older coral groups that have since gone extinct 3–5 . Here, we report the first measurements of nitrogen isotope ratios of coral-bound organic matter (CB-δ 15 N) in samples from Mid-Devonian reefs (Givetian, ca. 385 Ma), which represent the oldest isotopic constraint on the evolution of photosymbiosis to date. The colonial tabulate and loosely colonial (dendroid) corals analyzed have low CB-δ 15 N values (1.59‰±0.53‰ and 1.45±0.66‰, respectively) in comparison with co-occurring solitary rugose corals (4.38‰±1.34‰). The isotopic difference between Devonian colonial and solitary corals (2.86‰±0.86‰) is statistically indistinguishable from that observed between modern symbiont-bearing and symbiont-barren corals (3.38‰±1.05‰). From this evidence we infer that Mid-Devonian colonial tabulate and loosely colonial (dendroid) corals hosted active photosymbionts, while solitary rugose corals did not. The low CB-δ 15 N values of the Devonian colonial tabulate and dendroid corals relative to the modern range suggest that Devonian reefs formed in biogeochemical regimes analogous to the modern oligotrophic subtropical gyres. Widespread oligotrophy during the Devonian likely may have promoted coral photosymbiosis, potentially explaining why Devonian reefs were the most productive reef ecosystems of the Phanerozoic.

The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ15 N) using the oxidation-denitrifier (O-D) method provides a novel tool to study past changes in N cycling processes.METHODS:We report a set of methodological improvements to the O-D method, including (a) a method for sealing the reaction vials in which the oxidation of organic N to NO3 - takes place, (b) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO3 - to N2 O, and (c) a method for storing recrystallized dipotassium peroxodisulfate (K2 S2 O8 ) under Ar atmosphere.RESULTS:The new sealing method eliminates the occasional contamination and vial breakage that occurred previously while increasing sample throughput. The protocol for bypassing pH adjustment does not affect BB-δ15 N, and it significantly reduces the processing time. Storage of K2 S2 O8 reagent under Ar atmosphere produces stable oxidation blanks over more than 3.5 years. We report analytical blanks, accuracy, and precision for this methodology from eight users over the course of ~3.5 years of analyses at the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by low N content (0.30 ± 0.13 nmol N, 1σ, n = 195) and stable δ15 N (-2.20 ± 3.13‰, n = 195). The analysis of reference amino acid standards USGS 40 and USGS 65 indicates an overall accuracy of -0.23 ± 0.35‰ (1σ, n = 891). The analysis of in-house fossil standards gives similar analytical precision (1σ) across a range of BB-δ15 N values and biominerals: zooxanthellate coral standard PO-1 (6.08 ± 0.21‰, n = 267), azooxanthellate coral standard LO-1 (10.20 ± 0.28‰, n = 258), foraminifera standard MF-1 (5.92 ± 0.28‰, n = 243), and tooth enamel AG-Lox (4.06 ± 0.49‰, n = 78).CONCLUSIONS:The methodological improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs; however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotope composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel) records diet and trophic position. The δ15Nenamel of modern African mammals shows a 3.7‰ increase between herbivores and carnivores as expected from trophic enrichment, and there is a strong positive correlation between δ15Nenamel and δ15Nbone-collagen values from the same individuals. Additionally, δ15Nenamel values of Late Pleistocene fossil teeth preserve diet and trophic level information, despite complete diagenetic loss of collagen in the same specimens. We demonstrate that δ15Nenamel represents a powerful geochemical proxy for diet that is applicable to fossils and can help delineate major dietary transitions in ancient vertebrate lineages.

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

Along the coastal Tropical Eastern Pacific (TEP), regions of strong seasonal upwelling bring cold, nutrient-rich waters, controlling ecological conditions and sustaining millions of people through large-scale fisheries. The TEP is also important for the regulation of global climate and is affected by large-scale environmental processes such as ENSO. How the nutrient dynamics of this region will respond to climate change and what the implications will be for coastal ecology remains unknown. Environmental records are needed that capture intra and inter-decadal variation and extend over millennia where these biotic and abiotic processes interact. Here, we develop a new sampling approach and construct two coral skeleton records (n = >600) from reef matrix cores that extend six millennia, from the upwelling Gulf of Panamá and the non-upwelling Gulf of Chiriquí. We ask what effects millennial-scale climate patterns have on upwelling in the region, and how the magnitude of upwelled nutrients influences ecological productivity and even human habitation. We combined multiple proxies using climatic (carbonate δ18O), nutrient (skeletal-organic matrix δ15N), diagenetic (taphonomic scoring), ecological (benthic community composition), and temporal (U-Th dates) data. Using Generalised Additive Models to assess variability, we find strong divergences in the nutrient (δ15N; range >5 ‰) records between Gulfs, while δ18O (range ~2‰) is more stable. The greatest variation in δ15N values occurs during times of high reef accretion whereas δ18O is constant, suggesting that nutrients, not temperature, are driving reef productivity. Taphonomic, taxonomic, and age data reveal periodic shifts and collapses of coral communities that differ between Gulfs. We end by drawing connections between these ecological shifts to the episodic human habitation documented during the late-Holocene and hypothesize what this may mean for ecosystem resilience and environmental management under future climate.

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (delta N-15, delta O-18, delta(15-18) = delta N-15-delta O-18) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate delta N-15 is lower (4.9-5.8 parts per thousand) than the sub-thermocline source, Subantarctic Mode Water (6.9 parts per thousand); we attribute this difference to local N-2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N-2 fixation rate of 7-25 Tg N.a(-1), with the upper limit likely biased high. In the mixed layer, nitrate delta N-15 and delta O-18 rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N-2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate delta(15-18) is low, due to both N-2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-delta(15-18) nitrate in Agulhas rings indicates leakage of low-delta N-15 nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Fossil‐bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil‐bound N isotopic composition of different fossil types, including deep and shallow water scleractinian corals, foraminifera, diatoms and tooth enamel. Our experiments show that exposure to different oxidizing reagents does not significantly affect the N isotopic composition or N content of any of the fossil types analyzed, demonstrating that organic matter is well protected from changes in the surrounding environment by the mineral matrix. In addition, we show that partial dissolution (of up to 70%–90%) of fossil aragonite, calcite, opal, or enamel matrixes has a negligible effect on the N isotopic composition and N content of the fossils. These results suggest that the isotopic composition of fossil‐bound organic material is relatively uniform, and also that N exposed during dissolution is lost without significant isotopic discrimination. Finally, our heating experiments show negligible changes in the N isotopic composition and N content of all fossil types at 100°C. At 200°C and hotter, any N loss and associated nitrogen isotope changes appear to be directly linked to the sensitivity of the mineral matrix to thermal stress, which depends on the biomineral type. These results suggest that, so long as high temperature does not compromise the mineral structure, the biomineral matrix acts as a closed system with respect to N, and the N isotopic composition of the fossil remains unchanged.

The analyses of the stable isotope ratios of carbon (δ 13 C), nitrogen (δ 15 N), and oxygen (δ 18 O) in animal tissues are powerful tools for reconstructing the feeding behavior of individual animals and characterizing trophic interactions in food webs. Of these biomaterials, tooth enamel is the hardest, most mineralized vertebrate tissue and therefore least likely to be affected by chemical alteration (i.e., its isotopic composition can be preserved over millions of years), making it an important and widely available archive for biologists and paleontologists. Here, we present the first combined measurements of δ 13 C, δ 15 N, and δ 18 O in enamel from the teeth of modern fauna (herbivores, carnivores, and omnivores) from the well-studied ecosystem of Gorongosa National Park (GNP) in central Mozambique. We use two novel methods to produce high-precision stable isotope enamel data: (i) the “ oxidation-denitrification method,” which permits the measurement of mineral-bound organic nitrogen in tooth enamel (δ 15 N enamel ), which until now, has not been possible due to enamel’s low organic content, and (ii) the “ cold trap method ,” which greatly reduces the sample size required for traditional measurements of inorganic δ 13 C enamel and δ 18 O enamel (from ≥0.5 to ≤0.1 mg), permitting analysis of small or valuable teeth and high-resolution serial sampling of enamel. The stable isotope results for GNP fauna reveal important ecological information about the trophic level, dietary niche, and resource consumption. δ 15 N enamel values clearly differentiate trophic level (i.e., carnivore δ 15 N enamel values are 4.0‰ higher, on average, than herbivores), δ 13 C enamel values distinguish C 3 and/or C 4 biomass consumption, and δ 18 O enamel values reflect local meteoric water (δ 18 O water ) in the park. Analysis of combined carbon, nitrogen, and oxygen stable isotope data permits geochemical separation of grazers, browsers, omnivores, and carnivores according to their isotopic niche, while mixed-feeding herbivores cannot be clearly distinguished from other dietary groups. These results confirm that combined C, N, and O isotope analyses of a single aliquot of tooth enamel can be used to reconstruct diet and trophic niches. Given its resistance to chemical alteration, the analysis of these three isotopes in tooth enamel has a high potential to open new avenues of research in (paleo)ecology and paleontology.

Abstract Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs, however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotopic composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel) records diet and trophic position in modern and fossil ecosystems. The δ15Nenamel of modern African mammals shows a trophic enrichment of 3.7 ‰ between herbivores and carnivores, as well as a strong positive correlation between δ15Nenamel and δ15Nbone-collagen values from the same individuals. δ15Nenamel values of Late Pleistocene fossil teeth record expected dietary patterns, despite complete diagenetic loss of collagen in the same specimens. We demonstrate that δ15Nenamel represents a powerful new paleodietary proxy that could help delineate major dietary transitions in ancient vertebrate lineages, such as the onset and intensification of animal resource use in early hominins.

Nitrogen isotope ratios (delta N-15) are a well-established tool for investigating the dietary and trophic behavior of animals in terrestrial and marine food webs. To date, delta N-15 values in fossils have primarily been measured in collagen extracted from bone or dentin, which is susceptible to degradation and rarely preserved in deep time (>100,000 years). In contrast, tooth enamel organic matter is protected from diagenetic alteration by the mineral structure of hydroxyapatite and thus is often preserved over geological time. However, due to the low nitrogen content (<0.01 %) of enamel, the measurement of its nitrogen isotopic composition has been prevented by the analytical limitations of traditional methods. Here, we present a novel application of the oxidation-denitrification method that allows measurement of delta N-15 values in tooth enamel (delta N-15(enamel)). This method involves the oxidation of nitrogen in enamel-bound organic matter to nitrate followed by bacterial conversion of nitrate to N2O, and requires >= 100 times less nitrogen than traditional approaches. To demonstrate that delta N-15(enamel) values record diet and trophic behavior, we conducted a controlled feeding experiment with rats and guinea pigs (n = 37). We determined that nitrogen concentration in tooth enamel (x(-) = 5.0 + 1.0 nmol/mg) is sufficient for delta N-15(enamel) analyses with >= 5 mg untreated enamel powder. The nitrogen isotope composition of enamel reflects diet with an enrichment (Delta N-15(enamel-diet)) of ca. 2-4 parts per thousand. delta N-15(enamel) values differ significantly between dietary groups and clearly record a shift from pre-experimental to experimental diet. The small sample size required (<= 5 mg) by this method permits analyses of sample size-limited, diagenetically robust tooth enamel, and, as such it represents a promising new dietary proxy for reconstructing food webs and investigating the trophic ecology of extant and extinct taxa.

Perhaps the most important feedback to orbital climate change is CO2 storage in the deep ocean. By regulating atmospheric CO2, ocean carbon storage synchronizes glacial climate in both hemispheres, and drives the full magnitude of glacial-interglacial climate change. However few data exist that directly track the deep ocean’s carbon chemistry over a glacial cycle. Here, we present geochemical reconstructions of deep ocean circulation, redox, and carbon chemistry from sediment cores making up a detailed depth profile in the South Atlantic, alongside a record of Southern Ocean surface water CO2, spanning the last glacial cycle. These data indicate that initial glacial CO2 drawdown is associated with a major increase in surface ocean pH in the Antarctic Zone of the Southern Ocean, cooling at depth, enhanced deep ocean stratification, and carbon storage. Deep ocean carbon storage and deep stratification are further enhanced when CO2 falls at the onset of Marine Isotope Stage 4, and are also pronounced during the LGM, illustrating a link between orbital scale climate stages and deep ocean carbon. However our data also illustrate non-linear feedbacks to orbital forcing during glacial terminations, which show abrupt decreases in pH in Southern Ocean surface and subsurface waters, as CO2 is rapidly expelled from the deep ocean at the end of the last ice age.