Extreme lowering of deglacial seawater radiocarbon recorded by both epifaunal and infaunal benthic foraminifera in a wood-dated sediment core

Abstract

Abstract. For over a decade, oceanographers have debated the interpretation and reliability of sediment microfossil records indicating extremely low seawater radiocarbon (14C) during the last deglaciation – observations that suggest a major disruption in marine carbon cycling coincident with rising atmospheric CO2 concentrations. Possible flaws in these records include poor age model controls, utilization of mixed infaunal foraminifera species, and bioturbation. We have addressed these concerns using a glacial–interglacial record of epifaunal benthic foraminifera 14C on an ideal sedimentary age model (wood calibrated to atmosphere 14C). Our results affirm – with important caveats – the fidelity of these microfossil archives and confirm previous observations of highly depleted seawater 14C at intermediate depths in the deglacial northeast Pacific.

Anomalous > 2000‐Year‐Old Surface Ocean Radiocarbon Age as Evidence for Deglacial Geologic Carbon Release

“…The Gulf of California core site is bathed in the subsurface, northward-flowing Mexican Coastal Current (MCC in Figure 1). The MCC is also the source of the California Undercurrent (Gómez-Valdivia et al, 2015) that overlies sediment core sites along the Pacific margin of Baja California with comparable 14 C records from benthic forams (orange diamonds in Figure 1A) (Lindsay et al, 2015;Marchitto et al, 2007;Rafter et al, 2018). The similarity in the benthic 14 C measurements from these geographically distinct "Gulf" and "Undercurrent" sites over the past 30-kyr (shown as both 14 C ages and Δ 14 C in Figure 3) is striking and provides strong evidence for the persistence of modern circulation pathways (including the estuarine Gulf circulation) over the past 30-kyr .…”

“…In other words, the symbols in Figure 4 represent measurements minimally influenced by bioturbation or where bioturbation would work against the observed benthic-planktic age reversals. Given that the modern deep-surface seawater 14 C age difference near our core site of +539 ± 43 years ( Figure 1C) includes some known (but not quantified) flux of geologic carbon and partial equilibration of surface water with the atmosphere, the benthic-planktic 14 C age differences in Figure 4B suggest significantly larger geologic carbon flux (Reimer et al, 2013)), benthic forams (lines; (Lindsay et al, 2016;Marchitto et al, 2007;Rafter et al, 2018)), and planktic forams (symbols) are shown in two ways: (A) As a 14 C age and (B) as Δ 14 C. Measurements from the Gulf of California sediment (black; this study) are compared with those from the California Undercurrent site (orange (Lindsay et al, 2015;Marchitto et al, 2007)) (see Figure 1 for locations). The Gulf sediment core age model is constrained by wood 14 C ages (brown symbols) calibrated to atmosphere 14 C/C.…”

“…We present the differencing of these benthic-planktic 14 C ages alongside atmospheric CO 2 concentrations and the δ 18 O and δ 13 C values of G. bulloides in Figure 4. White diamonds represent foraminifera abundance maxima in both the benthic and planktic species (from Rafter et al, 2018), and blue circles represent samples where bioturbation biases the benthic-planktic 14 C age difference toward high values (down in Figure 4B). In other words, the symbols in Figure 4 represent measurements minimally influenced by bioturbation or where bioturbation would work against the observed benthic-planktic age reversals.…”

“…For example, if we arbitrarily remove the benthic/planktic pair with the largest 14 C age reversal (i.e., at 15-kyr BP), the estimated deglacial Q added drops to 0.76 Tmol C yr -1 (0.01 Pg C yr -1 ). However, our mass balance calculation (equations 2 and 3) underestimates Q added because it ignores air-sea gas exchange as well as the potentially elevated DIC levels in the inflowing water (equation 1) that are implied by the interpretation of low 14 C/C benthic observations in the region (Lindsay et al, 2015;Marchitto et al, 2007;Rafter et al, 2018).…”

“…Importantly, since our benthic and planktic forams are picked from the same sediment intervals, the 14 C age differences do not depend on the actual calendar ages: hence, this approach is independent of any age model errors. Nevertheless, we have excellent age model control using calibrated 14 C dates of terrestrial material (microscopic wood fragments) recovered from the core (see Rafter et al, 2018, for a detailed discussion).…”