Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial

Biological productivity and carbon export in the equatorial Atlantic are thought to have been dramatically higher during the last glacial period than during the Holocene. Here we reconstruct the pH and CO2 content of surface waters from the eastern equatorial Atlantic Ocean over the past ~30 k.y. using the boron isotope composition of Globigerinoides ruber (a mixed-layer–dwelling planktic foraminifera). Our new record, combined with previously published data, indicates that during the last glacial, in contrast to today, a strong west to east gradient existed in the extent of air:sea equilibrium with respect to pCO2 (∆pCO2), with the eastern equatorial Atlantic acting as a significant source of CO2 (+100 μatm) while the western Atlantic remained close to equilibrium (+25 μatm). This pattern suggests that a fivefold increase in the upwelling rate of deeper waters drove increased Atlantic productivity and large-scale regional cooling during the last glacial, but the higher than modern ∆pCO2 in the east indicates that export production did not keep up with enhanced upwelling of nutrients. However, the downstream decline of ∆pCO2 provides evidence that the unused nutrients from the east were eventually used for biologic carbon export, thereby effectively negating the impact of changes in upwelling on atmospheric CO2 levels. Our findings indicate that the equatorial Atlantic exerted a minimal role in contributing to lower glacial-age atmospheric CO2. INTRODUCTION Over at least the past 800 k.y., the CO2 content of the atmosphere has shifted from ~240– 280 ppm during the warm interglacial periods to 180–200 ppm during the cold glacial periods (Petit et al., 1999; Lüthi et al., 2008); most attention has been focused on the amount of carbon stored in the deep ocean during glacials to explain these CO2 changes (e.g., Sigman et al., 2010). One important mechanism in this regard is the biological pump: biomass produced in the surface ocean sinks to depth and decomposes, thereby pumping both nutrients and organic carbon into the deep ocean, where the carbon is sequestered away from the atmosphere and the nutrients are temporarily unavailable to fuel new biological production. Given their importance today in terms of oceanic primary production (Fig. 1B), attention has long focused on the equatorial oceans to at least partially explain the lower glacial CO2 levels (e.g., Mix, 1989). A small proportion of the equatorial regions is termed high-nitrate, low-chlorophyll (HNLC; e.g., the eastern equatorial Pacific Ocean), where the plentiful supply of macronutrients (N, P) from upwelling of cold (Fig. 1A), nutrient-rich (Fig. 1C) and high-CO2 (Fig. 1D) water is often underutilized because of the relative paucity of essential micronutrients such as Fe (Moore et al., 2013). The incomplete utilization of macronutrients in HNLC regions gives rise to outgassing of excess CO2 to the atmosphere (Fig. 1D). Changing the efficiency of nutrient utilization (e.g., through enhanced productivity via dust fertilization of Fe-limited areas; Martin, 1990) clearly has the potential to lower the pCO2 sw (sw is seawater) in these regions. However, most areas of the equatorial and low-latitude oceans are non-HNLC regions, where productivity is not micronutrient limited (e.g., by Fe), and organisms are eventually able to fully utilize all the available macronutrients (N, P; Fig. 1C) with correspondingly lower quantities (5–30 μatm) of excess CO2 (defined herein as ∆pCO2 = pCO2 sw – pCO2 atm; e.g., the eastern equatorial Atlantic; Fig. 1D). As a result, across the lowlatitude non-HNLC regions macronutrient (as opposed to micronutrient) limitation currently exists (Moore et al., 2013). In these regions, there is more limited scope to decrease pCO2 sw by enhanced nutrient utilization, since most of the nutrients are already nearly fully utilized. Despite the apparent potential for changing pCO2 sw in either HNLC or non-HNLC regions via alleviation of macronutrient or micronutrient limitation, it has been hypothesized that their role in glacial-interglacial CO2 change should have been minor (Sigman et al., 2010; Hain et al., 2014), because of the following: (1) Nutrients supplied to the low-latitude surface from below are all eventually consumed by productivity as the nutrient-rich water flows away from the site of upwelling before the water is able to sink into the ocean interior (Sigman and Haug, 2003). Even if an increase in on-axis (where axis refers to the longitudinally extensive zone of upwelling) consumption of nutrients occurred in an equatorial upwelling region (e.g., due to a relief from Fe limitation), leading to a local decrease in pCO2 sw, this may not change atmospheric CO2 significantly because off-axis productivity may be correspondingly reduced, causing little net change in the residual unused nutrient concentration when the water ultimately descends back into the ocean interior. This almost complete utilization of available nutrients is in stark contrast to higher latitudes, such as the Southern Ocean, where the utilization of nutrients is currently inefficient (and consequently is associated with outgassing of CO2), but may have been more efficient during glacial times (e.g., Sigman et al., 2010). (2) The high-latitude surface ocean is in direct communication with the deep ocean, whereas the low-latitude surface ocean is not, so changes in low-latitude pCO2 sw are hypothesized to be somewhat buffered from driving atmospheric CO2 changes because of the much larger size of the high-latitude/deep-ocean reservoir (Broecker et al., 1999). One region of the oceans that has welldocumented changes in surface ocean productivity and export production across glacialinterglacial cycles is the equatorial Atlantic (Bradtmiller et al., 2007; Kohfeld et al., 2005, and references therein, and their figure 2c). While this has been interpreted as evidence of a strengthening of the biological pump and hence driving a portion of glacial CO2 drawdown (e.g., Mix, 1989), this remains to be quantitatively tested. Here we use the boron isotopic composition (expressed in terms of δ11B) of mixed-layerdwelling planktic foraminifera Globigerinoides ruber to reconstruct surface water pH and therefore pCO2 sw across a transect of sites spanning the equatorial Atlantic Ocean over the past ~30 k.y., allowing us to directly test the role of this region in glacial-interglacial pCO2 atm variations.