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

38 Publications
Applied Filters: First Letter Of Last Name: S Reset
Recent evidence from the sulfur isotopic record indicates a transition ∼2.5 billion years ago from an ocean chemistry first dominated by iron and then by sulfide. It has been hypothesized that the selection of metal centers in metalloenzymes has been influenced by the availability of metals through geological time, in particular as a result of…
Throughout the year 2001, aerosol samples were collected continuously for 10 to 15 days at the French Antarctic Station Dumont d Urville (DDU) (66°40′S, 140°01′E, 40m above mean sea level). The nitrogen and oxygen isotopic ratios of particulate nitrate at DDU exhibit seasonal variations that are among the most extreme observed for nitrate on Earth…
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…

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…

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…

We have previously argued that the Antarctic and subarctic North Pacific are stratified during ice ages, causing to a large degree the observed low CO2 levels of ice age atmospheres by sequestering respired CO2 in the ocean abyss. Here, we suggest a mechanism for the major deglaciations of the late Pleistocene. The mechanism begins with…

Global climate and the atmospheric partial pressure of carbon dioxide (pco2atm) are correlated over recent glacial cycles, with lower pco2atm during ice ages, but the causes of the pco2atm changes are unknown. The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 leak was…
We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ15N and δ18O in the ocean interior. The δ18O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ15N and δ18O of mean ocean nitrate by…
Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the δ15N and δ18O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in δ18O than in δ15N as one ascends from abyssal waters into the denitrification-impacted…
This article outlines the isotope systematics of the major processes in the ocean nitrogen (N) cycle, their impact on the isotopic compositions of the major N reservoirs in the ocean, and how the N isotopes in the modern ocean and sedimentary record may record current N fluxes and past changes. The dominant dynamic is of kinetic isotope…
Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3-) 15N/14N and 18O/16O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ18O is up to 3%o greater than in δ15N, whereas our…
The low-latitude ocean is strongly stratified by the warmth of its surface water. As a result, the great volume of the deep ocean has easiest access to the atmosphere through the polar surface ocean. In the modern polar ocean during the winter, the vertical distribution of temperature promotes overturning, with colder water over warmer, while the…
[1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ̃ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to…
Using an ocean box model, we have studied the effect of altered circulation on the oceanic distributions of phosphate (PO4-3) and the 13C/12C and 14C/12C of dissolved inorganic carbon to evaluate competing hypotheses for the cause of observed nutrient depletion and 13C enrichment at intermediate depths of the Atlantic during the last ice age…
We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N2O) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and…
Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during the ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean s biological pump , the sequestration of carbon in the ocean interior by the…
We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0…
Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a diatom-bound N fraction which is 0-4‰ lower in δ15N than the bulk sediment, typically 3‰ lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and δ15N…
We investigate the response of the calcite lysocline to changes in the export production of the low-latitude surface ocean (the combined equatorial, tropical, and subtroical regions). We employ different CaCO3 throughput schemes in a time-dependent ocean carbon cycle model to separate the CaCO3 production/iysocline balance from the other…
We have adapted the ammonia diffusion method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devarda s alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass…
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)…
Deep (>0.8km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO 3 -, up to 50mM N 2 (90 times air saturation at surface) and 1 to 400μM NH 3/NH 4 +. To determine whether the oxidized N species were introduced by mining activity, by recharge of…
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…
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…
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…
We report wintertime nitrogen and oxygen isotope ratios (δ15N and δ18O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea ice edge. We focus here on the Antarctic region (south of 50.3°S), where application…
Surface nitrate concentration is a potentially useful diagnostic in reconstructing the past circulation of high-latitude North Atlantic waters. Moreover, nutrient consumption in the North Atlantic surface impacts the atmospheric concentration of carbon dioxide. To reconstruct nutrient conditions in the subpolar North Atlantic region during the…
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…
In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as fixed nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial-interglacial changes in…
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…
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…
In a sediment core from the Pacific sector of the Antarctic Zone (AZ) of the Southern Ocean, we report diatom-bound N isotope (δ15Ndb) records for total recoverable diatoms and two distinct diatom assemblages (pennate and centric rich). These data indicate tight coupling between the degree of nitrate consumption and Antarctic climate across the…
The nitrogen isotope composition of diatom opal (δ15Ndb) is a valuable recorder of nitrate utilization in the polar surface oceans and a measure of the efficiency of the biological pump. Past down-core records of δ15Ndb involved the measurement of the biogenic opal fraction up to 150μm in size, which should represent the bulk of the preserved…
Coincident with the intensification of Northern Hemisphere Glaciation (NHG) around 2.73 million years (Ma) ago, sediment cores from both the open subarctic North Pacific and the Antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed, representing one of the most abrupt and dramatic changes in the marine sediment record…

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…