Increasing primary productivity in the oligotrophic Tethyan coastal ocean during the Paleocene-Eocene warming episode

Highlights

• Response of the oligotrophic coastal ocean during global warming is grossly unknown.

• Tethyan low-latitude, shallow-marine sections record PETM in carbonate sequence.

• Primary productivity (coralline algae) increased during the PETM despite low nutrient.

• The higher atmospheric pCO₂ might have triggered the excessive growth of algae.

• Oligotrophic coastal ocean can act as an effective carbon sink.

Abstract

The coastal upwelling zones, occupying only ~0.5% of the global ocean, account for ~10% of the global primary productivity. The CO₂ fixation by primary producers amplifies in the upwelling zones during global warming due to the higher nutrient supply. Based on the presumption that the nutrient-deficient coastal ocean is less productive, the state of the oligotrophic coastal ocean is often neglected in the productivity-climate change studies. The present study investigated the changes in the primary productivity, redox condition, and nutrient content, using algal abundance, total organic carbon, and various major, trace, and rare earth elements and yttrium (REY) proxies, of the oligotrophic equatorial eastern Tethyan coastal ocean across the Paleocene-Eocene Thermal Maximum (PETM), a prominent paleo-global warming event. Despite the lower nutrient (lower Ni_EF, Cu_EF, and Zn_EF) contents, and invariable salinity, pH, and light conditions, the PETM interval shows extensive growth of coralline red algae in the hypoxic-oxic water column. Based on these observations, and inferences drawn from the previous laboratory experiments, conducted on the algal growth in varying pCO₂ by others, we postulate that the increased atmospheric CO₂ concentrations during the PETM probably enhanced the primary productivity of the oligotrophic Tethyan coastal ocean. If so, then the oligotrophic coastal ocean may be considered as an effective CO₂ sink and likely to play a pivotal role in carbon cycle-climate connection studies.

Introduction

About half of the total primary production on the Earth is contributed by the tiny phytoplankton in the oceans (Antoine et al., 1996; Field et al., 1998; Falkowski, 2012). The phytoplanktons fix a humongous amount of atmospheric CO₂via. photosynthesis and play a vital role in controlling the global carbon cycle and climate (Field et al., 1998). Acting as an effective CO₂ sink, their abundance is crucial during global warming, when there is excess CO₂ in the atmosphere. However, the warmer oceans during global warming are likely to hinder the phytoplankton growth by increasing the ocean thermal stratification and reducing the vertical mixing of the nutrients from the sub-surface layers to the upper layer of the ocean (Riebesell et al., 2000; Polovina et al., 2008; Boyce et al., 2010). On the contrary, phytoplanktons are likely to flourish in the coastal oceans, especially in the upwelling zones, where the intensified wind (due to the enhanced land-sea temperature gradients) accelerate the coastal upwelling and thereby, the nutrient content of the coastal waters (Bakun, 1990; Gregg et al., 2005; Boyce et al., 2010; Sydeman et al., 2014; Xiu et al., 2018). Thus, the upwelling zones, covering ~0.5% of the global ocean, probably act as major CO₂ sinks during global warming episodes (Bauer et al., 2013).

Except for the upwelling zones, significant areas of the coastal ocean are nutrient deficient or oligotrophic. Since oligotrophic coastal oceans are less productive in terms of phytoplankton growth, they have been grossly overlooked in the carbon cycle and climate change studies. Therefore, it is not well known whether the oligotrophic coastal ocean would remain less productive or become fertile during the global warming episodes (Barnet et al., 2020). The Paleocene-Eocene boundary (~56 Ma) witnessed one such extreme short-lived (~170 ± 30 Ka) warming episode (5–10 °C), popularly known as the Paleocene-Eocene Thermal Maximum (PETM; Dunkley Jones et al., 2013; Zeebe and Lourens, 2019; Stokke et al., 2020; Teng et al., 2021). The addition of an enormous amount of depleted greenhouse gases (CO₂ and CH₄) to the ocean-atmosphere system, recorded as negative carbon isotope excursions (CIE) in the ocean and terrestrial sediments, caused the warming during the PETM (see McInerney and Wing, 2011 and reference therein). In this backdrop, the present study is focused to understand the state of the oligotrophic, equatorial, eastern Tethyan coastal ocean, surrounding the Indian sub-continent (NE India; Fig. 1), across the PETM warming interval, using primary and export productivity (algal abundance, TOC_DC, and Ba_bio), redox condition (Ce/Ce*, Mo_EF and U_EF) and nutrient supply (Ni_EF, Cu_EF, and Zn_EF) proxies.