David C. Lund

Southwest Atlantic water mass evolution during the last deglaciation

The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5–17.5u2009kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate 13C-depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300u2009m remained 13C-depleted until 15u2009kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15u2009kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000u2009m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Evidence for a biological pump driver of atmospheric CO2 rise during Heinrich Stadial 1

The initial trigger of the atmospheric CO2 rise during Heinrich Stadial 1 (HS1: 14.5–17.5u2009kyr B.P.) remains elusive. We present a compilation of four paired surface and intermediate-depth foraminiferal δ13C records to test whether reduced biological pump efficiency led to the initial CO2 rise during the last deglaciation. Surface ocean δ13C decreased across HS1 while intermediate-depth δ13C increased, leading to a reduction in the upper ocean δ13C gradient. Our compilation also suggests the δ13C gradient increased during the Bolling-Allerod (12.9–14.5u2009kyr B.P.) and decreased during the Younger Dryas (YD: 11.7–12.9u2009kyr B.P.). The HS1 and YD data are consistent with reduced biological export of isotopically light carbon from the surface ocean and its remineralization at depth. Our results support the idea that a weaker Atlantic Meridional Overturning Circulation decreased biological pump efficiency by increasing the overall fraction of preformed nutrients in the global ocean, leading to an increase in atmospheric CO2.

Enhanced δ13C and δ18O Differences Between the South Atlantic and South Pacific During the Last Glaciation: The Deep Gateway Hypothesis

Enhanced vertical gradients in benthic foraminiferal δ13C and δ18O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North-Atlantic sourced interior waters; nonetheless our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high-resolution depth transects of Cibicidoides spp. δ13C and δ18O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between ~1.0 and 2.5 km was more than 1 ‰ enriched in δ13C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ18O was ~ 0.5 ‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a ‘deep gateway’ effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east-west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern-sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.

Carbon storage in the mid-depth Atlantic during millennial-scale climate events

Carbon isotope minima were a ubiquitous feature of the mid-depth Atlantic during Heinrich Stadial 1 (HS1, 14.5-17.5 kyr BP) and the Younger Dryas (YD, 11.5-12.9 kyr BP) yet their cause remains unclear. Recent evidence indicates that North Atlantic processes triggered the δ13C anomalies, with weakening of the Atlantic Meridional Overturning Circulation (AMOC) being the most likely driver. Model simulations suggest slowing of the AMOC increases the residence time of mid-depth waters in the Atlantic, resulting in the accumulation of respired carbon. Here we assess ΣCO2 storage in the South Atlantic using benthic foraminiferal B/Ca, a proxy for [CO32-]. Using replicated high resolution B/Ca records from ~ 2 km water depth on the Brazil Margin, we show that [CO32-] decreased during HS1 and the YD, synchronous with apparent weakening of the AMOC. The [CO32-] response is smaller than in the tropical North Atlantic during HS1, indicating there was a north-south gradient in the [CO32-] signal similar to that for δ13C. The implied variability in ΣCO2 is consistent with model results, suggesting that carbon is temporarily sequestered in the mid-depth Atlantic during millennial-scale stadial events. Using a carbon isotope mass balance, we estimate that approximately 75% of the HS1 δ13C signal at the Brazil Margin was driven by accumulation of remineralized carbon, highlighting the non-conservative behavior of δ13C during the last deglaciation.

Anomalous Pacific‐Antarctic Ridge Volcanism Precedes Glacial Termination 2

We present results from a well‐dated sediment core on the Pacific‐Antarctic Ridge (PAR) that document a ∼15 cm thick layer of basaltic ash shards that precedes the penultimate deglaciation (Termination 2). The glasses have MORB composition consistent with an axial source and their morphologies are typical of pyroclastic deposits created by submarine volcanism. The ash layer was deposited ∼7 km from the PAR axis, a distance that implies buoyant plumes lofted debris high into the water column with subsequent fallout to the core location. We infer plume rise height using grain settling velocities, the water depth at the core site, and deep ocean current speeds from ARGO floats. Rise heights of 1.5 km or less require unrealistically large current speeds to transport grains to the core site. Instead, the data are consistent with a plume rise height of at least 2 km, implying that T2 was an interval of anomalous volcanism along this segment of the PAR. The timing and duration of the ash deposit is consistent with glacial‐interglacial modulation of ridge magmatism. Volcaniclastic records from additional locations will be necessary to assess whether the PAR record is a rare find or it is representative of mid‐ocean ridge volcanism during glacial terminations.