Phytoplankton dynamics and nitrogen cycling during Oceanic Anoxic Event 2 (Cenomanian/Turonian) in the upwelling zone of the NE proto-North Atlantic

The Cretaceous is considered being one of the warmest periods in the Mesozoic, with low-latitudinal (<30°) sea surface temperatures being warmer than modern-day temperatures by >5 °C (Clarke and Jenkyns, 1999; Wilson et al., 2002; Clayson et al., 2016; O'Brien et al., 2017; Scotese et al., 2021). Within the Cretaceous the most pronounced warming events occurred across the Cenomanian/Turonian boundary (CTB; ∼94 Ma) (Forster et al., 2007; Meyers et al., 2012; O'Brien et al., 2017; Robinson et al., 2019) and further coincided with Oceanic Anoxic Event 2 (OAE 2) that marked a period of ocean deoxygenation and intense organic carbon burial in marine sediments (Schlanger and Jenkyns, 1976; Schlanger et al., 1987; Jenkyns, 2010). Global-scale environmental change that occurred in relation to intensified volcanisms and paleoceanographic changes (e.g., Li et al., 2022; Jones et al., 2023), profoundly impacted on biogeochemical cycles and marine ecosystems (e.g., Tsikos et al., 2004; Du Vivier et al., 2015; Owens et al., 2016; Jenkyns et al., 2017; Erba et al., 2019; Sweere et al., 2020a, Sweere et al., 2020b; Yobo et al., 2021).

Substantial changes have been documented in abundances and diversity of marine microfossils, such as coccolithophorids (Erba et al., 2019) and dinoflagellates (van Helmond et al., 2014; Dodsworth and Eldrett, 2019), representing important marine primary producers in the Cretaceous oceans (Falkowski et al., 2004). These phytoplankton groups, however, represent only a small part of the marine primary producer community, which dominantly lacks hard parts and therefore has a poor preservation potential. Accordingly, trends in phytoplankton community dynamics during Cenomanian-Turonian warm phase are poorly constrained. Improved understanding of marine phytoplankton community structures and dynamics under warm climates is of major interest to predict effects of modern-day warming on marine primary producers (e.g., Behrenfeld et al., 2006; Hallengraeff, 2010; Winder and Sommer, 2012). In particular, the role of the major marine nutrient nitrogen and its relationship with shifts between eukaryotic and prokaryotic phytoplankton groups is subject of ongoing debate (e.g., Rau et al., 1987; Kuypers et al., 2004; Sepúlveda et al., 2009; Higgins et al., 2012; Ruvalcaba Baroni et al., 2015; Owens et al., 2016; Naafs et al., 2019).

Here, we use chemotaxonomically diagnostic molecular fossils (hopanoid, steroid biomarkers) that can be linked to different phytoplankton groups to assess changes in the community structure of marine primary producers in the Tarfaya Basin (NE proto-North Atlantic Ocean) during the Cenomanian and lowermost Turonian, including Cretaceous OAE 2. Application of molecular fossils allows a holistic assessment of phytoplankton community dynamics in the geological past (e.g., Huang and Meinschein, 1979; Peters et al., 2005; Schwark and Empt, 2006; Ruebsam et al., 2022a). Molecular geochemical data are combined with stable isotope data (C, N) to assess the relationship between environmental conditions, ecosystems and biogeochemical cycles. We here present the first study from the southern proto-North Atlantic that combines the different analytical approaches and thereby provides vital insights into phytoplankton community dynamics and nitrogen cycling not only during OAE 2, but also throughout the entire Cenomanian.

Study site

The Tarfaya-Laayoune Basin extends along the west coast of northern Africa. During the Late Cretaceous the basin was located at the eastern margin of the proto-North Atlantic (Figs. 1a, b). Upper Cretaceous sediment successions consist of organic-rich pelagic limestones and marlstones deposited in an unrestricted outer shelf setting in proximity to the shelf break (Wenke, 2014). The paleo-shelf was characterized by a ramp-like structure gently dipping towards the proto-North Atlantic to the

Organic and inorganic carbon burial

Samples analyzed from core Tarfaya SN°4 show TOC contents in the range 1.2–11.8 wt%, with most samples yielding TOC contents >3 wt%. Increases in the TOC content from about 2.5 to 5.1 wt% and from 6.0 to 11.8 wt% occur in the MCE and OAE 2 intervals, respectively. Both intervals mark carbon burial events that are accompanied by positive δ¹³C excursions (Fig. 2) (Takashima et al., 2009; Jenkyns, 2010). During OAE 2, the most significant TOC increase occurred during the main phase (b in Fig. 2),

Conclusions

Bulk and isotope geochemical proxies in combination with molecular fossils, diagnostic for marine phytoplankton groups revealed vital insights into organic matter accumulation, nitrogen cycling and phytoplankton community dynamics throughout the Cenomanian and lowermost Turonian at the Tarfaya shelf. Substantial enrichment of well-preserved marine organic matter that occurred throughout the middle-upper Cenomanian and lower Turonian, and peaked during OAE 2, can be related to a shift towards

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Financial support by the German Research Foundation (Schw554-25, Schw554-29) is gratefully acknowledged. Constructive comments by Stéphane Bodin and an anonymous reviewer are highly appreciated.