Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum

Abstract

The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ∼400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2−39.9 Ma), we documented sea-surface temperatures of 32−36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2−3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ∼8.1 Tg C yr−1, comprising ∼450 Pg C over this ∼55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate. Moreover, this study highlights the importance of the interplay between global climate and regional oceanic gateway evolution in determining local climate and oceanographic change.

Effects of redox variability and early diagenesis on marine sedimentary Hg records

“…7). Such a process is an important consideration for Hg studies that attempted to constrain volcanic activity during periods of extensive anoxia such as the Mesozoic OAEs (e.g., Percival et al, 2015;Scaife et al, 2017;Percival et al, 2021b), or similar Paleozoic events (e.g., Pisarzowska et al, 2020;Rakociński et al, 2022), but also more regional or local deoxygenation, as recognized during the Cenozoic (e.g., Jones et al, 2019;Cramwinckel et al, 2022). The lowered Hg/TOC also provides some constraints on a fraction of Hg that would have been buried with the same amount of TOC assuming stable Hg/ TOC, a "Hg deficit" relative to more oxygenated conditions.…”

Revisiting the Geographical Extent of Exceptional Warmth in the Early Paleogene Southern Ocean

“…The %brGMGTs relative to total brGMGTs and brGDGTs, the brGMGTI, and HMBT acyclic have all been proposed as temperature proxies (Table 1). Even though several recent papers have reported the occurrence of these compounds in Paleogene marine sediments (Bijl et al., 2021; Cramwinckel et al., 2022; Sluijs et al., 2020), their deep‐time application remains largely untested. We here explore the temperature‐proxy potential of brGMGTI, HMBT acyclic and %brGMGTs in our shallow‐marine setting (Figure 2, Figures S4–S6 in Supporting Information ).…”

“…The Point Margaret section yields a suite of brGMGTs that have recently been documented in mineral soils and river sediment (Kirkels et al, 2022), peats (Naafs, McCormick, et al, 2018;Tang et al, 2021), Paleogene lignites (Naafs, McCormick, et al, 2018), African lakes (Baxter et al, 2019) and both modern marine surface (Liu et al, 2012) and Paleogene marine sediments (Bijl et al, 2021;Cramwinckel et al, 2022;Sluijs et al, 2020). Naafs, McCormick, et al (2018) show that the relative abundance of brGMGTs over regular brGDGTs is positively correlated with MAAT in peats.…”

North Atlantic surface ocean warming and salinization in response to middle Eocene greenhouse warming

“…At southern high latitudes, where the event was first described (9), 4°to 6°C of warming has been estimated for both the sea surface and the deep sea, on the basis of oxygen isotope ratios of biogenic calcite (δ 18 O c ) and distributions of alkenones (U K′ 37 ) and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs; TEX 86 ) (9,10,19,24). A rise in sea surface temperature (SST) of similar magnitude has subsequently been observed at mid-latitude sites in the former Tethys Ocean (25)(26)(27)(28) and in the North, South, and equatorial Atlantic oceans (3,10,(29)(30)(31)(32)(33). However, with some exceptions, most inferences of MECO SST warming are based on δ 18 O c records of foraminifera or bulk carbonate, which are also affected by (changes in) the oxygen isotope composition of surface ocean seawater (δ 18 O sw ).…”