Periodic mid‐Cretaceous oceanic anoxic events linked by oscillations of the phosphorus and oxygen biogeochemical cycles

Abstract

[1] A series of oceanic anoxic events (OAEs) occurred in the mid-Cretaceous warm period (120-80 Ma) that have been linked with high rates of organic carbon burial, warm high-and low-latitude temperatures, and sea-level changes. OAEs have been studied individually, but a causal mechanism that connects them has been lacking. We show that peaks in phosphorus accumulation in marine sediments broadly coincide with OAEs 1a, 1b, 1d, 2, and 3, and exhibit a 5-6 Myr periodicity, which for reactive-P is prominent over 100-80 Ma. Oxic-anoxic oscillations of this frequency are also found in a model of the coupled N, P, C, and O 2 biogeochemical cycles. These oscillations are maintained by positive feedbacks between phosphate concentration, biological productivity, and anoxia in the global ocean and counteracting, but slower, negative feedbacks involving changes in atmospheric oxygen. An increase in phosphorus weathering rate above a critical threshold can shift the system into self-sustaining oscillation. This could have been caused by tectonic and volcanic forcing increasing atmospheric CO 2 and global warmth 120-80 Ma, augmented by the rise of flowering plants around 100 Ma. With a plausible forcing scenario, we are able to reproduce the approximate timing of OAEs 1a, 1b, 1d, 2, and 3 in the model.

Sulfide induces phosphate release from polyphosphate in cultures of a marine Beggiatoa strain

“…Massive phosphorite deposits from the geological past usually formed in organic matter-rich sediments such as black shales (Piper and Codispoti, 1975) and are often associated with oceanic anoxic events (Handoh and Lenton, 2003). This supports our assumption that phosphorites formed preferentially in sediments with high sulfate reduction rates.…”

A safe operating space for humanity

“…This suggests that for humans to trigger an OAE should still be over 1,000 years away, thus shifting it down the list in our current sphere. Our tentative modelling analyses (using the model by Handoh and Lenton, 2003) show that a 10-fold increase of P inflow to the oceans (i.e., slightly higher than the current level), if sustained for 1,000 years, would raise the anoxic fraction of the ocean from 0.14 to 0.22. Current estimates of available phosphate rock reserves (up to 20 Gt of P) suggest that such an input could not be sustained for more than 1000 years.…”

“…The crossing of a critical threshold of P inflow to the oceans has been suggested as the key driver behind global-scale ocean anoxic events (OAE), potentially explaining past mass extinctions of marine life (Handoh and Lenton 2003). The dynamics between bi-stable oxic and anoxic conditions is believed to be induced by positive feedbacks between anoxia, phosphorus recycling from sediments and marine productivity.…”

“…Modelling suggests that a sustained increase of phosphorus inflow to the oceans exceeding 20% of the natural background weathering rate could have been enough to induce past OAEs (Handoh and Lenton, 2003). Assuming a relatively low estimate of "pre-agricultural" P-input to the oceans of 1.1 Mt yr -1 (3.5 E10 mol P yr -1 ), this increased inflow corresponds to only ~225,000 tonnes P yr -1 (0.72 E10 mol P yr -1 ).…”

Marine geochemical data assimilation in an efficient Earth System Model of global biogeochemical cycling

“…Thus, we are currently unable to assess feedback between bottom-water dysoxia and anoxia on global nutrient cycling, particularly involving phosphate, nitrate, and/or iron. For instance, O 2 -depleted bottom waters are know to be associated with enhanced phosphorous regeneration from marine sediments (Ingall and Jahnke, 1994), leading to the possibility of positive feedback between productivity and anoxia which might be central to the understanding of past Oceanic Anoxic Events (Van Cappellen and Ingall, 1994;Handoh and Lenton, 2003). Future developments to the GENIE-1 model will address the sedimentary control of nutrient cycling, as well as explicitly simulating sulphate reduction and sulphide release to the ocean, which currently we treat implicitly as a water-column process in order to avoid problems encountered in previous models when O 2 demand due to organic matter remineralisation exceeds dissolved O 2 availability Zhang et al, , 2003.…”