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

Abstract

Geologic carbon from seafloor volcanism may influence late Pleistocene glacial terminations by increasing the global inventory of the greenhouse gas CO 2 . However, the evidence for geologic carbon flux associated with deep sea volcanism has been, so far, equivocal. Here, we construct a regional, glacial-deglacial carbon budget of the volcanically active Gulf of California using microfossil 14 C measurements and find results consistent with an increased addition of geologic carbon related to local seafloor volcanism during the deglaciation. Our estimates point to enhanced geologic carbon flux both before and during the last deglaciation that generally occur alongside carbonate preservation. This leads us to suggest that the carbon was added in the form of partially neutralized, 14 C-free bicarbonate associated with known Gulf sedimentary processes-a carbon source that would have a minimal effect on atmospheric CO 2 . Plain Language SummaryWe account for the carbon entering and leaving the waters of the Gulf of California since the last ice age. Our results argue for increased supply of geologic carbon alongside enhanced volcanic activity after the last ice age. We argue that this delivery of geologic carbon to Gulf seawater was in the form of bicarbonate, not CO 2 , which would have a minimal impact on seawater acidity and is consistent with global sedimentary records.

Deglacial Ventilation Changes in the Deep Southwest Pacific

“…Sediment core age models may be substantially improved using wood remains and tephra layers, but such records are geographically limited (Rafter et al, 2018;Siani et al, 2013;Sikes et al, 2000;Skinner et al, 2015;Zhao & Keigwin, 2018). Regardless of materials used, recent studies show that deep-water radiocarbon reconstructions could be further complicated by possible release of 14 C-depleted carbon from local geological settings (Lizarralde et al, 2010;Rafter et al, 2019;Ronge et al, 2016;Stott et al, 2019).…”

Ideas and perspectives: Emerging contours of a dynamic exogenous kerogen cycle

“…Organic matter can be categorized into biospheric and lithogenic (i.e., kerogen) forms, with both exhibiting similar 13 C signatures yet modern and dead 14 C signatures, respectively (e.g., Lewan, 1986;Meyers, 1994). The constraints imposed by radiocarbon indicate there was a source of carbon to the atmosphere and ocean depleted in or devoid of radiocarbon (Broecker and Clark 2010;Hain et al, 2014;Rafter et al, 2019;Zhao et al, 2018), thereby limiting potential contributions from modern biospheric organic carbon sources. Other studies have proposed that carbon sourced from deep ocean DIC was the predominant source for carbon transferred to the atmosphere during glacial terminations (e.g., Hain et al, 2014).…”

“…This dilution of radiocarbon-dead CO 2 in the atmosphere may well have been complemented by other terrestrial sources such as the oxidation of subglacial paleosols and permafrost-bound organic carbon (Zeng, 2007;Simmons et al, 2016;Tesi et al, 2016;Crichton et al, 2016;Martens et al, 2020;Winterfeld et al, 2018;Lindgren et al, 2018;Köhler et al, 2014;Ciais et al, 2012) and by volcanic emissions triggered by deglacial unloading of the lithosphere (Roth and Joos, 2012). In addition to studies of weathering in glacial forefields and source-to-sink tracing of sedimentary kerogen, several lines of geochemical evidence including atmospheric carbon isotope composition ( 13 C and 14 C), which have thus far received contorted, partial explanations (Broecker and Clark, 2010;Schmitt et al, 2012;Broecker and McGee, 2013), glacial-interglacial changes in 14 C of DIC in seawater (Rafter et al, 2019;cf. discussions therein), seawater osmium isotope changes, and longterm atmospheric O 2 content, conceptually go hand in hand with an opening of the exogenous kerogen cycle modulated by glacial activity.…”

Ideas and perspectives: Emerging contours of a dynamic exogenous kerogen cycle

“…3;Smith et al, 1999;Schmitt et al, 2012), which is a strong indicator of respired organic carbon acting as source directly to the atmosphere (Bauska et al, 2016). Additionally, the constraints imposed by radiocarbon clearly indicate there must have been a source of carbon to the atmosphere and ocean depleted or devoid of radiocarbon (Broecker and Clark 2010;Hain et al, 2014;Rafter et al, 2019;Zhao et al, 2018), thereby limiting potential contributions from a modern biospheric organic carbon source. Other studies have proposed that carbon sourced from dissolved inorganic carbon from the deep ocean was the predominant source for carbon to the atmosphere at glacial terminations (e.g., Hain et al, 2014).…”

“…This dilution of radiocarbon-dead CO2 in the atmosphere may well have been complemented by other terrestrial sources such as subglacial paleosol oxidation (Zeng, 2007;Simmons et al, 2016), permafrost-bound organic carbon oxidation (Tesi et al, 2016;Winterfeld et al, 2018;Köhler et al, 2014;Ciais et al, 2012), and by (time-delayed) volcanic emissions due to unloading of the lithosphere (Roth and Joos, 2012). In addition to studies of weathering in glacial forefields and source-to-sink tracing of sedimentary kerogen, several lines of geochemical evidence including atmospheric carbon isotopic composition ( 13 C and 14 C), which have thus far received contorted, partial explanations (Broecker and Clark, 2010;Schmitt et al, 2012;Broecker and McGee, 2013), glacial-interglacial changes in 14 C of dissolved inorganic carbon in seawater (Rafter et al, 2019, c.f., discussions therein), seawater osmium isotope changes, and long-term atmospheric O2 content, conceptually go hand-in-hand with an opening of the exogenous kerogen cycle modulated by glacial activity. While geomagnetic variability and ocean ventilation together struggle to fully explain observed changes in atmospheric radiocarbon (Broecker and Barker, 2007;Cheng et al, 2018), the dilution of atmospheric CO2 by accelerated ancient terrestrial organic carbon oxidation at glacial terminations, in conjunction with other mechanisms including atmosphere-ocean gas exchange (e.g., Sigman et al, 2010;Marcott et al, 2014;Menviel et al, 2018;Martin, 1990;Sarntheim et al, 2013) appears as a simple and plausible explanation.…”