The sedimentary expression of oceanic anoxic event 1b in the North Atlantic

Abstract

The Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on palaeogeographical and depositional setting as well as on regional palaeoclimate. Marine sediments at a particular location may therefore be more or less organic‐rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic‐rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic‐rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic‐rich sediments from the shelf to deep sea depocentres as mud‐laden organic‐rich turbidites, especially in the northern North Atlantic. The so‐called ‘black shales’ are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms.

Transient cooling episodes during Cretaceous Oceanic Anoxic Events with special reference to OAE 1a (Early Aptian)

“…However, these processes seem not to have been accompanied by significant longterm increases in pCO 2 related to volcanogenic/hydrothermal activity, given that oxygen-isotope records from bulk pelagic carbonates from Italy and Switzerland and TEX 86 from the eastern North Atlantic show immediate post-OAE 1a trends to cooler temperatures. This pattern of climatic deterioration continued its irregular decline up to the latest Aptian before more variable temperatures, including pulses of warming, took hold around the stage boundary with the Albian (figure 2), when the regional organic-rich levels of OAE 1b were developed [18,19,23,38,43,50,[116][117][118][119][120][121]. Cooler intervals affecting bottom-water temperatures are suggested by the local presence of glendonites in Upper Aptian-Lower Albian sediments of the High Arctic and Australia [3,23,122,123].…”

Fossil‐Bearing Concretions of the Araripe Basin Accumulated During Oceanic Anoxic Event 1b

“…Enrichment factors of Mo, U, V (redox sensitive trace elements; RSTEs) peaked within interval S2 and recovered gradually within interval S3 (Figure 4). Increased EFs of RSTEs is a well‐known feature of sediments deposited under reducing bottom water conditions (e.g., Benamara et al., 2020; Charbonier et al., 2018; Trabucho Alexandre et al., 2011). This set of trace metals tend to be more soluble under oxidizing conditions and less soluble under reducing conditions.…”

“… (a) Global paleogeographic reconstruction at 113 Ma with location of discussed sites: (1) Sítio Sobradinho outcrop (Custódio et al., 2017; Bom et al., 2021, and this study), (2) Deep Sea Drilling Project (DSDP) Site 364, South Atlantic Ocean (Behrooz et al., 2018), (3) Gyabula Formation, Tibet (Li et al., 2016), (4) Linderos section, North Atlantic Ocean (Gutiérrez‐Puente et al., 2022), (5) Francizo Zarco Dam (FZD) section, North Atlantic Ocean (Nunez‐Useche et al., 2016), (6) DSDP Site 545, North Atlantic Ocean (Trabucho Alechandre et al., 2011), (7) Poggio le Guaine section, Tethys (Sabatino et al., 2015), (8) Vocontian Basin, Thetys (Benamara et al., 2020), (9) Borehole DY‐1, epicontinental sea (present‐day China; Xu et al., 2022). (b) Geological map and section of the Araripe Basin.…”

“…A combination of (a) extreme global warmth, (b) sluggish ocean circulation, accompanied by (c) water column stratification, and/or (d) enhanced primary productivity, is invoked to explain extensive black shale deposition during OAE‐1b in open marine environments, all leading to bottom water oxygen depletion (Erbacher et al., 2001). Climate change during OAE‐1b also intensified the hydrological cycle and, therefore, continental weathering and freshwater runoff (Leckie et al., 2002), favoring the burial and preservation of organic matter in lakes and ocean sediments (Trabucho Alexandre et al., 2011). However, the effects of these climatic and environmental changes on continental settings are still poorly understood, especially when considering records from the Southern Hemisphere (Li et al., 2016).…”

Carbon isotope stratigraphy of Torinosu‐type limestone in the western Paleo‐Pacific and its implication to paleoceanography in the Late Jurassic and earliest Cretaceous

“…Another potential tool for age determination is carbon isotope stratigraphy, which has been applied for inter-regional correlation (e.g. Padden et al 2002;Wissler et al 2003;Louis-Schmid et al 2007a;Ludvigson et al 2010;Alexandre et al 2011;Huck et al 2011;Najarro et al 2011;Hasegawa et al 2012;Stein et al 2012;Kakizaki et al 2013). In this study, we firstly improve the carbon isotope stratigraphy of the Torinosu-type limestone for a period of ca.…”