Ocean acidification and the Permo-Triassic mass extinction

Abstract

Ocean acidification triggered by Siberian Trap volcanism has been implicated as a kill18 mechanism for the Permo-Triassic mass extinction, but evidence for an acidification event 19 remains inconclusive. To address this, we present a high resolution seawater pH record across 20 this interval, utilizing boron isotope data combined with a quantitative modeling approach. In the 21 latest Permian, the alkalinity of the ocean increased, priming the Earth system with a low level of 22 atmospheric CO 2 and a high ocean buffering capacity. The first phase of extinction was 23 2 coincident with a slow injection of isotopically light carbon into the atmosphere-ocean, but the 24 ocean was well-buffered such that ocean pH remained stable. During the second extinction pulse, 25 however, a rapid and large injection of carbon overwhelmed the buffering capacity of the ocean, 26causing an abrupt and short-lived acidification event that drove the preferential loss of heavily 27 calcified marine biota. kyrs (2) and can be resolved into two distinct marine extinction pulses, with the respective kill 37 mechanisms appearing to be ecologically selective (3). The first occurred in the latest Permian 38 (Extinction Pulse 1; EP1) and was followed by an interval of temporary recovery before the 39 second pulse (EP2) which occurred in the earliest Triassic. The direct cause of the mass 40 extinction is widely debated with a diverse range of overlapping mechanisms proposed, 41 including widespread water column anoxia (4), euxinia (5), global warming (6) and ocean 42 acidification (7). 43Models of PTB ocean acidification suggest that a massive, and rapid, release of CO 2 from 44 Siberian Trap volcanism, acidified the ocean (7). Indirect evidence for acidification comes from 45 the interpretation of faunal turnover records (3, 8), potential dissolution surfaces (9) and Ca 46 3 isotope data (7). A rapid input of carbon is also potentially recorded in the negative carbon 47 isotope excursion (CIE) that characterizes the PTB (10, 11) . The interpretation of these records 48 is, however, debated (12), and of great importance to understanding the current threat of 49 anthropogenically-driven ocean acidification (11). 50Here, we test the ocean acidification hypothesis by presenting a novel proxy record of 51 ocean pH across the PTB, using the boron isotope composition of marine carbonates ( 11 additional counterbalancing alkalinity flux. This is consistent with independent proxy data (6). 129The alkalinity source may have been further increased through soil loss (26) carbon to the atmosphere, yet remarkably, the acidification event occurs after the decline in  13 C, 139when  13 C has rebounded somewhat and is essentially stable (Fig. 2). 140Unlike the first carbon injection, the lack of change in  13 C at this time rules out very 141 13 C-depleted carbon sources, because no counterbalancing strongly 13 C-enriched source exists.

Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation

“…Comparatively higher and more spatially variable OC loading of the sea floor in the South China basin under these circumstances might be triggered by enhanced primary productivity or OC sinking fluxes (Algeo et al, 2011. On the other hand, the depositional sites found in Iran lack evidence for bottom water anoxia and favour other drivers behind the marine faunal extinction, e.g., thermal stress or ocean acidification Schobben et al, 2014;Clarkson et al, 2015).…”

Boron Isotopes in the Ocean Floor Realm and the Mantle

“…However, several studies have used marine carbonate to reconstruct seawater d 11 B for various stages during the late Neoproterozoic and the Phanerozoic ( Fig. 8.7; Joachimski et al 2005;Kasemann et al 2010;Foster et al 2012;Clarkson et al 2015;Anagnostou et al 2016).…”

“…The stippled line is interpolated for periods where no model exists. Reconstruction of seawater d 11 B based on data from marine carbonate are marked in red boxes (Joachimski et al 2005;Kasemann et al 2010;Foster et al 2012;Clarkson et al 2015;Anagnostou et al 2016). Minimum and maximum seawater values for Eocene estimates are marked by upward-and downward-pointing triangles, respectively.…”

Understanding, Characterizing, and Communicating Responses to Ocean Acidification: Challenges and Uncertainties

“…Substantial changes are observed in some present-day coral reef and temperate benthic communities exposed to elevated CO 2 at ocean CO 2 seeps (e.g., Fabricius et al, 2011;see Box 2). Past OA episodes in the geological record are often marked by major extinction events of marine species, though OA typically occurred in conjunction with other major environmental changes, including climate change (Hönisch et al, 2012;Clarkson et al, 2015). …”