Studies in the Modern Ocean

CTD/hydrocast package descending into the Southern Ocean. Photo Kenemy

The CTD/hydrocast package descending into the Southern Ocean water column from the deck of the South African research icebreaker S.A. Agulhas II, sailing from Cape Town to the Antarctic winter ice edge. Photo: Preston Cosslett Kemeny ’15

 

References

220 Publications
Previous studies suggest that meridional migrations of the Antarctic Circumpolar Current may have altered wind-driven upwelling and carbon dioxide degassing in the Southern Ocean during past climate transitions. Here, we report a quantitative and continuous record of the Antarctic Circumpolar Current latitude over the last glacial-interglacial…
The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ15N) 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…
The nitrogen isotopic composition (15N/14N ratio, or δ15N) of enameloid-bound organic matter (δ15NEB) in shark teeth was recently developed to investigate the biogeochemistry and trophic structures (i.e., food webs) of the ancient ocean. Using δ15NEB, we present the first nitrogen isotopic evidence for trophic differences between shark taxa from a…
All else equal, if the ocean s “biological [carbon] pump” strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean s biological pump stored more carbon in the ocean interior during the…
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 δ15N and δ18O measurements of groundwater nitrate, we demonstrate the role of brackish…
The Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient-limited…
Studies have suggested that cancerous tissue has a lower 15N/14N ratio than benign tissue. However, human data have been inconclusive, possibly due to constraints on experimental design. Here, we used high-sensitivity nitrogen isotope methods to assess the 15N/14N ratio of human breast, lung, and kidney cancer tissue at unprecedented spatial…

The oxygen content of the oceans is susceptible to climate change and has declined in recent decades1, with the largest effect in oxygen-deficient zones (ODZs)2 , that is,mid-depth ocean regions with oxygen concentrations <5 μmol kg −1 (ref. 3). Earth-system-model simulations of climate warming predict that ODZs will expand until at least…

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 …

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 (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating…

It is understood that the global mean ocean nitrate δ15N is set by the δ15N of the input of fixed nitrogen (N) to the ocean (mostly N2 fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate…

The cyclic growth and decay of continental ice sheets can be reconstructed from the history of global sea level. Sea level is relatively well constrained for the Last Glacial Maximum (LGM, 26,500 to 19,000 y ago, 26.5 to 19 ka) and the ensuing deglaciation. However, sea-level estimates for the period of ice-sheet growth before the LGM vary by …

The analyses of the stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and oxygen (δ18O) 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…
The modern Pacific Ocean hosts the largest oxygen-deficient zones (ODZs), where oxygen concentrations are so low that nitrate is used to respire organic matter. The history of the ODZs may offer key insights into ocean deoxygenation under future global warming. In a 12-My record from the southeastern Pacific, we observe a >10 increase in…

Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate…

Dissolved oxygen (O2) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming1,2. However, models provide an unclear picture of future ODZ…

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…

The nitrogen isotopes (δ15N) in the organic fraction of accretionary hard part structures, such as fish otoliths, may provide life histories of dietary change. We performed controlled experiments to validate the dynamics of the isotopic signal incorporation into biominerals following dietary shifts and also compared whole-otolith and serial…

We present a dual-modulation Faraday rotation spectrometer with in-line optical subtraction for differential measurement of nitric oxide (NO) isotopologues. In-situ sample referencing is accomplished via differential dual-cell measurements, with 3.1 ppbv/rt(Hz) (15NO) sensitivity through 15 cm optical path length. Our system operates at 1.9x…

Background: Cancerous cells can recycle metabolic ammonium for their growth. As this ammonium has a low nitrogen isotope ratio (15N/14N), its recycling may cause cancer tissue to have lower 15N/14N than surrounding healthy tissue. We investigated whether, within a given tissue type in individual mice, tumoral and healthy tissues could be…
Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the…
Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present…

For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The δ13C of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water…

Bulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect…
Nitrogen isotope ratios (δ15N) are a well-established tool for investigating the dietary and trophic behavior of animals in terrestrial and marine food webs. To date, δ15N 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…
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)…
Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean s Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen…
Dissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ15N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r =…
(Figure Presented) Figure 4. Cycle of biologically driven N transformations that occur in natural and human-influenced terrestrial and marine environments. Nitrogen (N2) fixation (step 1) and N assimilation (from ammonium, nitrate, or organic N, step 2) are anabolic processes, whereas mineralization (step 3), nitrification (steps 4−6), DNRA (steps 7,…
Nitrogen (N) is used in many of life s fundamental biomolecules, and it is also a participant in environmental redox chemistry. Biogeochemical processes control the amount and form of N available to organisms ("fixed"N). These interacting processes result in N acting as the proximate limiting nutrient in most surface environments. Here, we review…
Historical coral skeleton (CS) δ18O and δ15N records were produced from samples recovered from sedimentary deposits, held in natural history museum collections, and cored into modern coral heads. These records were used to assess the influence of global warming and regional eutrophication, respectively, on the decline of coastal coral communities…
We present the first nitrogen isotope (δ15N) measurements of planktic foraminifera, paleoceanographically important zooplankton, from the nutrient-rich waters of the modern Southern Ocean. Foraminifera were collected from net tows in the Subantarctic and Polar Frontal Zones (SAZ and PFZ, respectively) south of Africa during winter 2015 and late…
Nutrient enrichment can slow growth, enhance bioerosion rates, and intensify algal competition for reef-building corals. In areas of high human population density and/or limited waste management, submarine groundwater discharge can transfer anthropogenic nutrients from polluted groundwater to coastal reefs. In this case study, we investigate the…
The15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may allow for reconstruction of fish trophic history and changes in food webs. To support this application, ground-truthing data are needed on the relationships among the δ15N of diet, of fish tissue (e.g., white muscle tissue, δ15Nwmt), and δ15Noto. Using a…
The shoaling of the Central American Seaway (CAS) around 4.6 Ma (million years ago) is thought to have enhanced the Gulf Stream, strengthening the Atlantic Meridional Overturning Circulation and potentially influencing the evolution of Pliocene climate. Paleoclimate records indicate a buildup of heat and salt in the Caribbean and changes in the…
Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ 15 N and δ 18 O) of nitrate before this nutrient is carried from the upper water column into the deep ocean by the formation of North Atlantic Deep Water (NADW). Here, nitrate δ 15 N and δ 18 O measurements along the 2013 CLIVAR/GO…
Both the nitrogen (N) isotopic composition (δ 15 N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the δ 15 N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested…
The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the 15N/14N ratio (d15N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma ago, d15N declined by 13 to 16 per mil in the…
From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5…
Most biological nitrogen transformations have characteristic kinetic isotope effects used to track these processes in modern and past environments. The isotopic fractionation associated with nitrogen fixation, the only biological source of fixed nitrogen (N), provides a particularly important constraint for studies of nitrogen cycling. Nitrogen…
This article outlines the isotope systematics of processes in the ocean nitrogen (N) cycle and their impact on the isotopic compositions of the major N forms, with an eye toward how the N isotopes may record current N fluxes and past changes. The dominant dynamic is of kinetic isotope discrimination, in which the light isotope of N (14N) is…

The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the…

In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom-bound 15 N/ 14 N (δ 15 N db ) and reduced export production during the ice ages indicates more complete nitrate (NO 3 − ) consumption. This evidence points to an ice age decline in gross NO 3 − supply from the deep ocean to the surface wind-mixed layer, which…
Planktonic foraminifer tests are major archives of environmental change and provide a multitude of proxies in paleoceanography and paleoclimatology. The application of such proxies is contingent upon a collaborative effort to better understand how the living organisms record the properties of their environment and how the resulting signals are…
Despite slow nutrient supply to the subtropical surface ocean, its rates of annual inorganic carbon drawdown and net oxygen production are similar to those of nutrient-rich high latitude waters. This surprisingly rapid carbon drawdown, if due to the production and export of marine biomass, cannot be explained in terms of known nutrient supply…
Human alteration of the global nitrogen cycle intensified over the 1900s. Model simulations suggest that large swaths of the open ocean, including the North Atlantic and the western Pacific, have already been affected by anthropogenic nitrogen through atmospheric transport and deposition. Here we report an ∼130-yearlong record of the 15N/14N of…
A rise in the atmospheric CO2 concentration of 20 parts per million over the course of the Holocene has long been recognized as exceptional among interglacials and is in need of explanation. Previous hypotheses involved natural or anthropogenic changes in terrestrial biomass, carbonate compensation in response to deglacial outgassing of oceanic…
We report the nitrogen (N) isotope ratios (δ15N) of planktic foraminifera collected from upper-ocean net tows (surface to 200 m), moored sediment traps, and core-top sediments at the Bermuda Time-series Site in the northern Sargasso Sea between 2009 and 2013. Consistent with previous measurements from low-latitude core-top sediments, the annually…
Nitrogen is a limiting nutrient for primary production in the western Arctic Ocean. Measurements of the nitrogen (15N/14N) and oxygen (18O/16O) isotope ratios of nitrate in the southeastern Beaufort Sea provide insight into biogeochemical cycling of nitrogen in the western Arctic Ocean. Nitrate O isotope ratios in the Pacific halocline evidence a…