Changes in the distribution of δ¹³C of deep water ΣCO₂ between the Last Glaciation and the Holocene

Abstract

Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean 513C values were 0.46 ø/00 lower during the last glacial maximum than during the Late IIolocene. The geographic distribution of 5•3C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high 513C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean 5•3C values during the glaciation are lower than both North Atlantic and Pacific 5•3C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched •3C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean. INTRODUCTIONThe biological and chemical processes that fractionate carbon isotopes in the ocean provide one of the most useful tracers for reconstructing past distributions of water masses and their properties. The present distribution of 5•3C of ECO2 reproduces the general distribution of water masses in the oceans, and the gradients in •3C between locations record the net flow direction between ocean basins. Certain species of benthic foraminifera (C. wuellerstorfi, in particular) faithfully record these gradients of 5•3C, providing the tool necessary to reconstruct the past distribution of 5•3C. In this paper, we present the 513C distribution for the ocean during the last 30,000 years, and from these data determine the consequences of glacial-interglacial climate change on thermohaline circulation. •3 C Distribution in the Ocean The distribution of •3C in the ocean is controlled principally by photosynthesis and remineralization of organic carbon, and by mixing between water masses of different isotopic composition. Photosynthesis in surface water preferentially extracts •2C from seawater, causing the enrichment of the surface water ZCO2 in •3C. The value of 5•3C in seawater, after primary producers have removed all nutrients, is controlled by the mean 513C and the mean nutrient concentration of the ocean [Broecker, 1982; Broecker and Peng, 1982]. Today when all of the nutrients are stripped from the surface water, the ZCO2 of the surface reservoir has lost approximately 10% of its total dissolved inorganic carbon. Since the carbon that has been removed by phytoplankton has a •3C equal to approximately-20 ø/00, the surface reservoir is enriched by 2.0 ø/00 over the mean 5•3C of the deep water (Figure 1). The mean ocean 5•3C value equals 0.0 %0 (PDB), so the surface value today equals approximately 2.0 ø/00 (PDB). Complete nutrient utilization by phytoplankton ...

Stable Isotope Record and Late Quaternary Sedimentation Rates at the Antarctic Continental Margin

“…Since these cha nges are respo nsible for only a minor part of the shift. the glacial/intergla cial S 13C-cycles are probably a function of mainly global changes in ocea n prod uctivity, nutrient and bottom Water mass d istribut ion (Curry et al, 1988;Duplessy et al, 1988 The diffe rence in 6 U e values of the bemhlc for aminifer E. exigua of abo ut 0.9°/00 be tween th e last glacial and late Holocen e time falls in th e same ran ge as calculated by Curry er al. (1988) for C. wuellersrorji £rom South Atlant ic co res (26°· 44°S).…”

The Holocene CO2 rise: Anthropogenic or natural?

“…With the onset of warm con ditions, the forests and bogs destroyed during glacial time reformed. On the basis of the 13 C record kept by the benthic foraminifera, this re-growth sequestered about 500 GtC [Curry et al, 1988] .The removal of this amount of C0 2 from the ocean-atmosphere reservoir would not only have lowered the atmosphere's C0 2 content but it would also have increased the carbonate ion content of ocean water. Such an increase would have deepened the transition zone that separates mid-depth sediment rich in CaC0 3 from abyssal sediment that had lost its CaC0 3 to dissolution.This downward shift would have unbalanced the ocean's carbon budget.…”

Indian Ocean circulation and productivity during the last glacial cycle

“…The δ 13 C of SK129−CR2 and ODP Site 849, both equatorial sites at the termination of two independent circulation and nutrient-regeneration pathways, also show comparable changes through time. The δ 13 C in equatorial Pacific Ocean sediment cores, including ODP Site 849, have been interpreted as reflecting changes in the mean ocean δ 13 C (Boyle, 1992;Curry, 1988;Duplessy et al, 1988). Based on the fact that SK129−CR2 tracks δ 13 C changes in ODP Site 849, we suggest that the deep equatorial Indian Ocean has undergone evolution in its deep-water δ 13 C that is similar to large parts of the deep Because the rapidity of circulation in the Southern Ocean ensures it has a uniform δ 13 C, this suggests a similar amount of carbon addition to the deep waters en-route to their respective equatorial termination points through nutrient regeneration.…”