Bruce Deck

Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica

A high-resolution ice-core record of atmospheric CO2 concentration over the Holocene epoch shows that the global carbon cycle has not been in steady state during the past 11,000 years. Analysis of the CO2 concentration and carbon stable-isotope records, using a one-dimensional carbon-cycle model,uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial-scale changes of atmospheric CO2 concentrations.

Dual modes of the carbon cycle since the Last Glacial Maximum.

The most conspicuous feature of the record of past climate contained in polar ice is the rapid warming which occurs after long intervals of gradual cooling. During the last four transitions from glacial to interglacial conditions, over which such abrupt warmings occur, ice records indicate that the CO2 concentration of the atmosphere increased by roughly 80 to 100 parts per million by volume (refs 1–4). But the causes of the atmospheric CO2 concentration increases are unclear. Here we present the stable-carbon-isotope composition (δ13CO2) of CO2 extracted from air trapped in ice at Taylor Dome, Antarctica, from the Last Glacial Maximum to the onset of Holocene times. The global carbon cycle is shown to have operated in two distinct primary modes on the timescale of thousands of years, one when climate was changing relatively slowly and another when warming was rapid, each with a characteristic average stable-carbon-isotope composition of the net CO2 exchanged by the atmosphere with the land and oceans. δ13CO2 increased between 16.5 and 9 thousand years ago by slightly more than would be estimated to be caused by the physical effects of a 5u2009°C rise in global average sea surface temperature driving a CO2 efflux from the ocean, but our data do not allow specific causes to be constrained.

Geochemistry and loading history of phosphate and silicate in the Hudson estuary

The loading history and geochemistry of soluble reactive phosphorus (SRP) and dissolved silica (DSi) are evaluated in the Hudson estuary using 16 years of axial transect data. SRP behaves atypically in the estuary. Profiles show conservative mixing between a large mid-salinity source and the freshwater and seaward end members. Order of magnitude calculations indicate that waste water treatment facilities (WWTFs) are the dominant mid-salinity SRP source. DSi profiles display behaviour more typical of other estuaries in the northeastern United States, showing conservative mixing during periods of high flow and a mid-salinity source during periods of low flow. A single layered multi-box model is used to evaluate the loading history of SRP and DSi. Shortly after the New York State phosphate detergent ban of 1972, the SRP load dropped to two-thirds of that typical of the early 1970s. Loading of SRP remained at this level until the mid-1980s when construction began at the largest point source. During the construction phase (1984–1986), SRP loading returned to the early 1970s level. Upon completion, the total load declined once again and by the end of the 1980s it reached a level approximately one-third of that existing prior to the detergent ban. Model calculations of observed DSi profiles do not show a similar time-trend. They suggest that during summer months dissolution of diatom tests is a major source of DSi; however, WWTF DSi loads also appear to be a significant source to the Hudson estuary.

A Record of Atmospheric Co2 During the Last 40,000 Years from the Siple Dome, Antarctica Ice Core

[1]xa0We have measured the CO2 concentration of air occluded during the last 40,000 years in the deep Siple Dome A (hereafter Siple Dome) ice core, Antarctica. The general trend of CO2 concentration from Siple Dome ice follows the temperature inferred from the isotopic composition of the ice and is mostly in agreement with other Antarctic ice core CO2 records. CO2 rose initially at ∼17.5 kyr B.P. (thousand years before 1950), decreased slowly during the Antarctic Cold Reversal, rose during the Younger Dryas, fell to a local minimum at around 8 kyr B.P., and rose continuously since then. The CO2 concentration never reached steady state during the Holocene, as also found in the Taylor Dome and EPICA Dome C (hereafter Dome C) records. During the last glacial termination, a lag of CO2 versus Siple Dome isotopic temperature is probable. The Siple Dome CO2 concentrations during the last glacial termination and in the Holocene are at certain times greater than in other Antarctic ice cores by up to 20 ppm (μmol CO2/mol air). While in situ production of CO2 is one possible cause of the sporadic elevated levels, the mechanism leading to the enrichment is not yet clear.

Record of cosmogenic in situ produced 14C in Vostok and Taylor Dome ice samples: Implications for strong role of wind ventilation processes

We present results of studies of in situ cosmogenic 14 C in several ice samples from the Vostok and Taylor Dome cores, spanning the time intervals of 20 and 11 k.y. B.P., respectively. The results are in variance with our findings for the Greenland Ice Sheet Project 2 (GISP 2) samples, where near quantitative 14 C retention was observed. The partitioning of 14 C in the CO and CO 2 phases is, however, quite similar in the GISP and two Antarctic ice samples. Noting that most of the in situ 14 C is produced in the ice during its accumulation to thicknesses of up to ∼10 m, we interpret the observed 14 C deficiencies in Antarctic ice samples as due to grain metamorphism (recrystallization and sublimation or evaporation caused by wind ventilation). Simplified models for wind ventilation exhibit an interesting feature of the firnification processes; they differently affect the concentrations of in situ 14 C and of the cosmogenic nuclides 10 Be and 36 Cl scavenged from the atmosphere in the accumulating firn. By studying the concentrations of in situ cosmogenic 14 C and the atmospheric cosmogenic nuclides in the same ice samples, one can hope to obtain fairly realistic models of the chemical impacts of firnification processes: specifically, relationships between precipitation and accumulation and the main processes contributing to modifications in the nuclide concentrations.

Feasibility of using sand dunes as archives of old air

Large unaltered samples of the atmosphere covering the past century would complement the history of atmospheric gases obtained from bubbles in ice cores, enabling measurement of geochemically important species such as O2, 14CH4, and 14CO. Sand dunes are a porous media with interstitial air in diffusive contact with the atmosphere, somewhat analogous to the unconsolidated layer of firn atop glaciers. Recent studies have demonstrated the value of firn as an archive of old air [Battle et al., 1996; Bender et al., 1994a]. Unlike firn, sand dunes are incompressible and so remain permeable to greater depths and may extend the firn record into the past century. To evaluate the feasibility of using sand dunes as archives of old air, we drilled 60 m deep test holes in the Algodones Dunes, Imperial Valley, California. The main objective was to see if the air in a sand dune is as old as predicted by a diffusion model, or if the dune is rapidly flushed by advective pumping during windstorms and barometric pressure changes. We dated the air with chlorofluorocarbons and krypton-85, anthropogenic tracers whose atmospheric concentrations are known and have been increasing rapidly in the past half century. These tracer data match the pure diffusion model well, showing that advection in this dune is negligible compared to diffusion as a transport mechanism and that the mean age of the air at 61 m depth is ∼10 years. Dunes therefore do contain old air. However, dunes appear to suffer from two serious drawbacks as archives. Microbial metabolism is evident in elevated CO2 and N2O and depressed CH4 and O2 concentrations in this dune, corrupting the signals of interest in this and probably most dunes. Second, isotopic analyses of N2 and O2 from the dune show that fractionation of the gases occurs due to diffusion of water vapor, complicating the interpretation of the O2 signal beyond the point of viability for an air archive. Sand dunes may be useful for relatively inert gases with large atmospheric concentration changes such as chlorofluorocarbons.

BOREAS TGB-06 Soil Methane Oxidation and Production from NSA BP and Fen Sites

The BOReal Ecosystem-Atmosphere Study Trace Gas Biogeochemistry (BOREAS TGB-6) team collected soil methane measurements at several sites in the Southern Study Area (SSA) and Northern Study Area (NSA). This data set contains soil methane consumption (bacterial CH4 oxidation) and associated C-13 fractionation effects in samples that were collected at various sites in 1994 and 1996 from enclosures (chambers). Methane C-13 data in soil gas samples from the NSA Young Jack Pine (YJP) and Old Jack Pine (OJP) sites for 1994 and 1996 are also given. Additional data on the isotopic composition of methane (carbon and hydrogen isotopes) produced in the NSA beaver ponds and fen bog in 1993 and 1994 are given as well. The data are stored in tabular ASCII files.