We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO(2) forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.

Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ13C transect of Duplessy et al. (1988). The distribution of δ13C of ΣCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end‐member δ13C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end‐member value of about 1.5‰, and a deep (>2000 m), southern source water with an end‐member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ18O. A revised glacial section of deep water δ13C documents the positions and gradients among these end‐member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.

Near-surface reaction of CO 2 -bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO 2 storage, and perhaps for direct capture of CO 2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes ("at the outcrop") rather than ex situ processes ("at the smokestack"). 545 Annu. Rev. Earth Planet. Sci. 2011.39:545-576. Downloaded from www.annualreviews.org by University of British Columbia on 10/27/12. For personal use only.

Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate reconstructions for decades. We use a new archive of climate, deep-sea corals, to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modern deepsea corals show strong linear trends between δ 13 C and δ 18 O. Slopes of these trends between samples are similar and range between 2.1-2.6 for ∆δ 13 C/∆δ 18 O. Linear trends intersect isotopic equilibrium for δ 18 O and are slightly depleted for δ 13 C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, fast precipitating bands, while isotopically enriched points correspond to slowly accumulating less dense aragonite. The densest, white band at the trabecular center is furthest from isotopic equilibrium for both carbon and oxygen. Data from this region fall off the linear trend between δ 18 O and δ 13 C. This deviation, where δ 13 C is anomalously heavy for the δ 18 O, does not support "vital effect" mechanisms that call upon kinetic fractionation to explain offsets from isotopic equilibrium. We propose a new mechanism for "vital effects" in biogenic carbonates that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region.

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 ...