Deglacial whole-ocean δ¹³C change estimated from 480 benthic foraminiferal records

Abstract

Terrestrial carbon storage is dramatically decreased during glacial periods due to cold temperatures, increased aridity, and the presence of large ice sheets on land. Most of the carbon released by the terrestrial biosphere is stored in the ocean, where the light isotopic signature of terrestrial carbon is observed as a 0.32-0.7‰ depletion in benthic foraminiferal δ 13 C. The wide range in estimated δ 13 C change results from the use of different subsets of benthic δ 13 C data and different methods of weighting the mean δ 13 C by volume. We present a more precise estimate of glacial-interglacial δ 13 C change of marine dissolved inorganic carbon using benthic Cibicidoides spp. δ 13 C records from 480 core sites (more than 3 times as many sites as previous studies). We divide the ocean into eight regions to generate linear regressions of regional δ 13 C versus depth for the Late Holocene (0-6 ka) and Last Glacial Maximum (19-23 ka) and estimate a mean δ 13 C decrease of 0.38 ± 0.08‰ (2σ) for 0.5-5 km. Estimating large uncertainty ranges for δ 13 C change in the top 0.5 km, below 5 km, and in the Southern Ocean, we calculate a whole-ocean change of 0.34 ± 0.19‰. This implies a terrestrial carbon change that is consistent with recent vegetation model estimates of 330-694 Gt C. Additionally, we find that a well-constrained surface ocean δ 13 C change is essential for narrowing the uncertainty range of estimated whole-ocean δ 13 C change.

Deglacial carbon cycle changes observed in a compilation of 117 benthic δ<sup>13</sup>C time series (20–6 ka)

“…This study seeks to improve our understanding of glacial- C of benthic foraminiferal calcite is a well-established carbon cycle proxy, which records the δ 13 C signature of the dissolved inorganic carbon (DIC) in seawater at seafloor depths (e.g., Woodruff and Savin (1985); Zahn et al (1986); Lutze and Thiel (1989); Duplessy et al (1988); Mackensen (2008); Gottschalk et al (2016); Schmittner et al (2017)). Averages of benthic foraminiferal δ 13 C time series, called stacks, can improve the signal-to-noise ratio of regional or global seawater changes (e.g., Lisiecki et al (2008);Lisiecki (2014)). Global mean benthic δ 13 C change is likely caused by changes in terrestrial organic 5 carbon storage (Shackleton, 1977; Curry et al, 1988; Duplessy et al, 1988; Ciais et al, 2012;Peterson et al, 2014), while vertical δ 13 C gradients may record changes in deep ocean carbon storage and atmospheric CO 2 (Oppo and Fairbanks, 1990; Flower et al, 2000; Hodell et al, 2003; Lisiecki, 2010).…”

“…), and proxies such as benthic δ 13 C, triple oxygen isotopes (Landais et al, 2007), and atmospheric carbonyl sulfide (Aydin et al, 2016). These methods produce estimates of change in terrestrial carbon storage between the LGM and Holocene that vary from 200-1900 PgC due to uncertainties and assumptions associated 5 with each method (see discussion and citations within Peterson et al (2014)). …”

“…Averages of benthic foraminiferal δ 13 C time series, called stacks, can improve the signal-to-noise ratio of regional or global seawater changes (e.g., Lisiecki et al (2008);Lisiecki (2014)). Global mean benthic δ 13 C change is likely caused by changes in terrestrial organic 5 carbon storage (Shackleton, 1977; Curry et al, 1988; Duplessy et al, 1988; Ciais et al, 2012;Peterson et al, 2014), while vertical δ 13 C gradients may record changes in deep ocean carbon storage and atmospheric CO 2 (Oppo and Fairbanks, 1990; Flower et al, 2000; Hodell et al, 2003; Lisiecki, 2010). The vertical δ Here we compile and analyze 117 high-resolution benthic δ 13 C records from the Atlantic, Pacific, and Indian Oceans spanning the last deglaciation to investigate changes in both the ocean and terrestrial biosphere components of the global carbon We analyze these stacks to test the following hypotheses:…”

“…C between the LGM and the Holocene is estimated to be 0.32‰ ± 0.20‰ (Peterson et al, 2014; Gebbie et al, 2015), but the timing of mean benthic δ 13 C change across the deglaciation is not well known. Deglacial changes in terrestrial carbon storage (soils and vegetation) can be reconstructed in many ways, including terrestrial vegetation proxies and archives (e.g., pollen, paleovegetation), carbon cycle models (e.g., box models, inverse methods, dynamic global vegetation models, biomization methods, etc.…”

“…We excluded any records with sample gaps of 4 kyr or larger and excluded any cores affected by the phytodetritus effect ("Mackensen effect") as assessed by the original authors and the criteria from Peterson et al (2014). We included one C. kullenbergi record from the deep South Atlantic (MD07-3076Q) (Waelbroeck et al, 2011) which may record a more negative δ…”