Bentho‐Pelagic Decoupling: The Marine Biological Carbon Pump During Eocene Hyperthermals

Abstract

The marine biological carbon pump sequesters carbon from the atmosphere into deep ocean waters and to a lesser extent, in seafloor sediments (Figure 1). Despite focused study, many questions remain regarding carbon cycle functioning and potential changes in a warming world-changes that will affect oceanic ecosystems and potentially serve as feedbacks to the climate-carbon cycle system (e.g., IPCC, 2014; Passow & Carlson, 2012). Today, the ocean sequesters a third to a quarter of anthropogenic carbon dioxide emissions, transferring carbon to the ocean's interior (e.g., Boscolo-Galazzo et al., 2018; IPCC, 2014). Changes in the efficiency of the carbon pump to store carbon in the ocean's interior affect the size of the oceanic pool of dissolved organic carbon (DOC), one of the Earth's largest pools of bioactive and exchangeable carbon, and thus are important for understanding potential carbon sequestration in the deep ocean (e.g., Ridgwell & Arndt, 2015). In past warm periods of Earth history (such as the Eocene) temperature dependency of the Abstract Future environmental change may profoundly affect oceanic ecosystems in a complex way, due to the synergy between rising temperatures, reduction in mixing and upwelling due to enhanced stratification, ocean acidification, and associated biogeochemical dynamics. Changes in primary productivity, in export of organic carbon from the surface ocean, and in remineralization deeper in the water column in the so-called "twilight zone" may substantially alter the marine biological carbon pump, thus carbon storage in the oceans. We present different proxy records commonly used for reconstructing paleoproductivity, and re-evaluate their use for understanding dynamic change within and between different constituents of the marine biological pump during transient global warming episodes in the past. Marine pelagic barite records are a proxy for carbon export from the photic and/or mesopelagic zone, and are not positively correlated with benthic foraminiferal proxies for arrival of organic matter to the seafloor over three early Eocene periods of global warming (Ocean Drilling Program Site 1263, SE Atlantic). These two proxies reflect processes in different parts of the water column, thus different components of the biological pump. An increase in temperature-dependent organic carbon remineralization in the water column would have caused decreased arrival of food at the seafloor, starving the benthic biota and explaining the differences between the proxies, and may have led to ocean deoxygenation. Carbon cycle modeling demonstrates the feasibility of enhanced water-column remineralization to explain both Site 1263 records, suggesting that this mechanism amplifies pCO 2 increase, representing a positive feedback during hyperthermal warming. Plain Language Summary A major challenge in understanding the response of the oceanic carbon cycle to ongoing global warming is predicting how much organic carbon is sequestered in the deep ocean waters, where it may remain for centuries, and in se...

Early Paleocene Paleoceanography and Export Productivity in the Chicxulub Crater

“…However, we know that temperatures were predominantly warm and stable throughout the Danian, with the only departure being the few degrees C of warming associated with the Dan-C2 event. Griffith et al (2021) examined changes in benthic foraminifera and barium flux across Eocene hyperthermals and found that warming caused increased remineralization of organic matter below the euphotic zone, recorded as an increase in Ba and a decline in benthic foraminifera. Unfortunately, the characteristic carbon isotope excursion associated with the Dan-C2 hyperthermal cannot be identified in our core, likely due to both the aforementioned diagenetic alteration and slow sedimentation rates.…”

“…Another possibility that must be addressed is the effect of temperature on metabolic rates of grazers, with warmer waters facilitating increased remineralization of sinking organic matter, essentially making the biological pump less efficient (e.g., Boscolo-Galazzo et al, 2021;Griffith et al, 2021). Unfortunately, pervasive diagenetic alteration of carbonates means that it is not possible to make meaningful interpretations of temperature trends at Site M0077 using the bulk carbonate oxygen isotope record.…”

Elevated Post K‐Pg Export Productivity in the Gulf of Mexico and Caribbean

“…[2021]). The amount of NPP that reaches the deep sea varies by region and is largely controlled by plankton ecology (Henson et al., 2012), but on the whole only 1%–3% of modern NPP reaches the deep ocean or sediments (e.g., de la Rocha and Passow, 2007; Griffith et al., 2021; Müller & Suess, 1979).…”

“…POM which sinks below the mesopelagic zone is effectively removed from the short-term carbon cycle, and so the export of organic matter below 1000 m is often referred to as "sequestration flux" (e.g., Passow and Carlson, 2012) or "transfer efficiency" (Henson et al, 2012). By this point, most remineralization has occurred, but POM sinking out of the mesopelagic zone has traveled through less than one third of the average depth of the ocean, and additional remineralization occurs all the way to (and at) the seafloor, before surviving POM is buried and removed from the carbon cycle on geologic time scales (referred to as "burial flux" by Griffith et al, 2021). The amount of net primary production that reaches the deep sea varies by region and is largely controlled by plankton ecology (Henson et al, 2012), but on the whole only 1-3% of modern net primary production reaches the deep ocean or sediments (e.g., Müller and Suess, 1979;de la Rocha and Passow, 2007;Griffith et al, 2021).…”

“…By this point, most remineralization has occurred, but POM sinking out of the mesopelagic zone has traveled through less than one third of the average depth of the ocean, and additional remineralization occurs all the way to (and at) the seafloor, before surviving POM is buried and removed from the carbon cycle on geologic time scales (referred to as "burial flux" by Griffith et al, 2021). The amount of net primary production that reaches the deep sea varies by region and is largely controlled by plankton ecology (Henson et al, 2012), but on the whole only 1-3% of modern net primary production reaches the deep ocean or sediments (e.g., Müller and Suess, 1979;de la Rocha and Passow, 2007;Griffith et al, 2021). Initial reconstructions of productivity change across the K-Pg boundary focused on carbonate proxies, specifically carbonate mass accumulation rates and carbon stable isotopes (e.g., Hsü and Mackenzie, 1985;Zachos et al, 1989).…”