Daniel M. Sigman

Glacial/interglacial variations in atmospheric carbon dioxide.

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the oceans ‘biological pump’, the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.

Dinitrogen fixation in the world's oceans

The surface water of themarine environment has traditionally beenviewed as a nitrogen (N) limited habitat, andthis has guided the development of conceptualbiogeochemical models focusing largely on thereservoir of nitrate as the critical source ofN to sustain primary productivity. However,selected groups of Bacteria, includingcyanobacteria, and Archaea canutilize dinitrogen (N2) as an alternativeN source. In the marine environment, thesemicroorganisms can have profound effects on netcommunity production processes and can impactthe coupling of C-N-P cycles as well as the netoceanic sequestration of atmospheric carbondioxide. As one component of an integrated ‘Nitrogen Transport and Transformations’ project, we have begun to re-assess ourunderstanding of (1) the biotic sources andrates of N2 fixation in the worldsoceans, (2) the major controls on rates ofoceanic N2 fixation, (3) the significanceof this N2 fixation for the global carboncycle and (4) the role of human activities inthe alteration of oceanic N2 fixation. Preliminary results indicate that rates ofN2 fixation, especially in subtropical andtropical open ocean habitats, have a major rolein the global marine N budget. Iron (Fe)bioavailability appears to be an importantcontrol and is, therefore, critical inextrapolation to global rates of N2fixation. Anthropogenic perturbations mayalter N2 fixation in coastal environmentsthrough habitat destruction and eutrophication,and open ocean N2 fixation may be enhancedby warming and increased stratification of theupper water column. Global anthropogenic andclimatic changes may also affect N2fixation rates, for example by altering dustinputs (i.e. Fe) or by expansion ofsubtropical boundaries. Some recent estimatesof global ocean N2 fixation are in therange of 100–200 Tg N (1–2 × 1014 g N)yr−1, but have large uncertainties. Theseestimates are nearly an order of magnitudegreater than historical, pre-1980 estimates,but approach modern estimates of oceanicdenitrification.

Influence of the intertropical convergence zone on the East Asian monsoon

The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

Spatial coupling of nitrogen inputs and losses in the ocean

Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world’s oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.

Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method

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 Devardas alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass fiber disk which is sandwiched between two porous Teflon membranes. We have investigated the conditions necessary to effect complete ammonia recovery from natural seawater samples and the use of Devardas alloy under these conditions. In addition, we have characterized the blanks in this method and designed a protocol to minimize them. Here, we report our protocol for nitrate extraction from seawater and provide an explanation of the protocol based on our method development work. To demonstrate the performance of the method, we present nitrate nitrogen isotopic data from nitrate standard additions to Sargasso Sea surface water and from several Southern Ocean depth profiles. The nitrate extraction method gives highly reproducible, complete recovery of nitrate and a standard deviation for isotopic analysis of < 0.2%c down to 5 μM nitrate (or lower). Replicate extractions of a nitrate standard added to Sargasso Sea surface water demonstrate agreement between the isotopic composition of the added and recovered N, with the extraction blank causing a ≤ 0.3%. discrepancy for 5 μM nitrate. The blanks inherent in the extraction procedure are from Devardas alloy and seawater dissolved organic nitrogen (“DON”). The N blank of the Devardas alloy reagent depends on brand and lot number. The Devardas alloy which we are currently using results in a blank of ~ 0.4 nmol N per 100 ml of seawater (effectively 0.4 μM). An isotopic correction is made for this blank. For standard incubation conditions, stored Woods Hole seawater (with ~ 10 μM DON) gives a ~ 0.6 μM DON blank, while stored Sargasso Sea (with ~ 6 μm DON) surface water gives a DON blank of 0.3–0.5 μM. The DON blank appears to cause the ≤ 0.3%. difference between the measured and actual isotopic composition of nitrate added to Sargasso Sea surface water at the 5 μM nitrate level. We discuss several ways to lower the DON blank for samples in which the DON concentration is high relative to the nitrate concentration. The nitrogen isotopic data from several Southern Ocean profiles, in conjunction with the other results presented in this paper, demonstrate the consistency of the data produced by the ammonia diffusion method. The ammonia diffusion-based protocol is more reliable and allows for better precision than the nitrate reduction/ammonia distillation method (Cline and Kaplan, 1975) in our hands. While the samples have an incubation time of 4 days or longer, we find that the diffusion method allows for higher throughput than the distillation method because samples can be run conveniently in large batches.

The polar ocean and glacial cycles in atmospheric CO2 concentration

Global climate and the atmospheric partial pressure of carbon dioxide () are correlated over recent glacial cycles, with lower during ice ages, but the causes of the changes are unknown. The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 ‘leak’ was stemmed during ice ages, increasing ocean CO2 storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO2 uptake. This explanation for lower ice-age , if correct, has much to teach us about the controls on current ocean processes.

The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific

The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO~ , and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is influenced seasonally by coastal upwelling. Nevertheless, sediment trap material evidently records the isotopic composition of new nitrogen sources, since average d15N is generally indistinguishable from d15N values for subsurface NO~ . Surficial sediments are also very similar to the average d15N value of the sediment traps, being within 1&. This di⁄erence in d15N between trap material and sediment is much less than the previously observed 4& di⁄erence for the deep sea. Better organic matter preservation at our margin sites is a likely explanation, which may be due to either low bottom O 2 concentrations or higher organic matter input to the sediments. All sites have d15N for sub-euphotic zone NO~ (8—10&) substantially elevated from the oceanic average (4.5—5&). This isotopic enrichment is a result of denitrification in suboxic subsurface waters (Gulf of California) or northward transport of denitrification influenced water (Monterey Bay and San Pedro Basin). Our results therefore suggest that downcore d15N data, depending on site location, would record the intensity of denitrification and the transport of its isotopic signature along the California margin. Temporal variations in d15N for the sediment traps do appear to respond to upwelling or convective injections of NO~ to surface waters as

Deglacial changes in ocean circulation from an extended radiocarbon calibration

Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to, about 11.8 kyr before present (BP), near the termination of the Younger Dryas cold period. If a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation, extending the well-calibrated radiocarbon timescale back further in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated corals and laminae counts in varved sediments, show conflicting results. Here we use radiocarbon data from varved sediments in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate and continuous radiocarbon calibration for the period 9 to 14.5 kyr BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that North Atlantic Deep Water formation shut down during the Younger Dryas period, but was gradually replaced by an alternative mode of convection, possibly via the formation of North Atlantic Intermediate Water.

Measuring 15N-NH4+ in marine, estuarine and fresh waters : An adaptation of the ammonia diffusion method for samples with low ammonium concentrations

Abstract We present a method for measuring 15 N –NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses large sample volumes (often 4 l) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured δ 15 N values.

The δ15N of nitrate in the southern ocean: Consumption of nitrate in surface waters

We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton. The δ15N of nitrate increases by 1–2‰ from deep water into the Antarctic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarctic depth profiles yields values in the range of 5–6‰ in east Indian sector and 4–5‰ in the east Pacific sector. Surface transect data from the Pacific sector also yield values of 4–5‰. The major uncertainty in the profile-based estimation of the isotope effect involves the δ15N of nitrate from the temperature minimum layer below the summertime Antarctic surface layer, which deviates significantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertically from the Subantarctic thermocline because of the distinctive relationships between the δ15N and concentration of nitrate in these two potential sources. Our Subantarctic samples, collected during the summer and fall, indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Pacific sector of the Subantarctic suggest an isotope effect of 4.5‰, indistinguishable from the Antarctic estimates in this sector.