Astronomically Driven Variations in Depositional Environments in the South Atlantic During the Early Cretaceous

Abstract

The extent and persistence of anoxia in the South Atlantic Ocean during its early opening phase in the Early Cretaceous is not well constrained, hindering a holistic understanding of the processes and mechanisms that drive past changes in water column redox conditions, as well as the impacts of such changes on marine ecosystems. Here we provide high‐resolution geochemical records from Deep Sea Drilling Project Site 364 that document variations in redox conditions, chemocline depth, marine productivity, and marine ecosystem dynamics in the northern South Atlantic during the Aptian. We show that many of these parameters varied across discrete sedimentary cycles expressed in the Deep Sea Drilling Project 364 succession. Our data indicate that during the initial stages of basin development, anoxic and euxinic conditions were prevalent and occasionally extended into the upper water column. However, strong cyclicity in sedimentological and geochemical parameters imply that the anoxia/euxinia was not a persistent state. We argue that the water column redox conditions during the Aptian were driven by changes in the hydrological cycle, induced by variations in astronomical forcing. We suggest that the episodically amplified hydrological cycle not only enhanced nutrient availability and marine productivity, but might also have caused density‐driven upper ocean stratification. The presence of black shales of similar age in other ocean basins suggests that this mechanism is broadly important for the formation of Early Cretaceous organic‐rich successions.

Driving mechanisms of organic carbon burial in the Early Cretaceous South Atlantic Cape Basin (DSDP Site 361)

“…In the Cape Basin, the occurrence of black shales (TOC >3%) is limited to a discrete time interval during the Early Aptian, followed by a gradual decrease in OC burial throughout the Aptian-Albian. A similar trend, though with a slightly different timing, has previously been observed at Site 364 located in the Angola Basin (Behrooz et al, 2018). These data consistently indicate that black shale occurrences in the emerging South Atlantic basins were amplified during early stages of basin https://doi.org/10.5194/cp-2020-15 Preprint.…”

“…Comparable alternations in Cretaceous black shale sequences have, however, been reported from both the North and South Atlantic throughout the Cretaceous, which have been attributed to geochemical expressions of large scale climate variations 505 driven by orbital forcing (Beckmann et al, 2005;Behrooz et al, 2018;Hofmann and Wagner, 2011;Wagner et al, 2013;Kolonic et al, 2005;Herrle et al, 2003). According to our simulations, Site 361 was located close to the subtropical descending limb of the paleo-Hadley Cell, a setting marked by strong latitudinal climatic gradients (Figure 10a).…”

“…(geological) time scale, OC burial was controlled by tectonic processes modifying basin geometry, marine gateway evolution, and ocean circulation (Donnadieu et al, 2016;Dummann et al, 2020;Arthur and Natland, 1979;Wagner and Pletsch, 1999), while shorter term (e.g. orbitally driven) changes in oceanic and atmospheric circulation triggered pronounced fluctuations in the magnitude of OC burial on time scales of ten to hundred thousand years (Beckmann et al, 2005;Behrooz et al, 2018;45 Hofmann and Wagner, 2011;Wagner et al, 2013;Kolonic et al, 2005;Herrle et al, 2003). During brief episodes ≤~1 Myr, marine black shales were deposited regionally to globally and accompanied by intense perturbations in ocean chemistry and ecology (Jenkyns, 2010;Weissert et al, 1998;Erba, 1994).…”

“…Each of the two sub-basins acted as a major depocenter for OC-rich 55 sediments (Zimmerman et al, 1987;Arthur and Natland, 1979;Macdonald et al, 2003) with exceptionally high OC burial rates in the Early Cretaceous, accounting for around 16% of global excess OC burial in an area that covered only around 1% of the total global ocean (McAnena et al, 2013). Data from Deep Sea Drilling Project (DSDP) Site 364, located at a paleolatitude of 25° S and an estimated paleo-water depth of ~300-400 m (Zimmerman et al, 1987) in the Angola Basin ( Figure 1), suggest that enhanced OC deposition occurred north of the Rio Grande Rise/Walvis Ridge during the Early to early Late 60 Aptian, sustained by the restricted basin geometry and the continuous supply of nutrients from surrounding continents (Arthur and Natland, 1979;Behrooz et al, 2018;Naafs and Pancost, 2014). In line with previous studies (e.g.…”

Reply to Hugh Jenkyns

“…These sections exhibit unusually high sedimentation rates and a carbon isotope record, which matches that of La Bédoule (Naafs et al, 2016, Supplementary Figure S1), underlining the completeness of both records. In the northern South Atlantic (e.g., DSDP Site 364 offshore Angola), an expanded sediment sequence showing evidence of cyclic episodes of intense anoxia/euxinia was deposited at neritic depths in a large, restricted basin (Behrooz et al, 2018;Kochhann et al, 2014), which may have contributed to organic carbon burial during C6 and C7. There is also evidence of black shale facies extending beyond isotope segment C6 in northeastern Tunisia (Elkhazri et al, 2013), albeit with TOC values in the upper Bedoulian black limestone facies (corresponding to segment C7) significantly lower (< 0.5 percent) than in the black limestones deposited within the C4 segment (4.5 percent).…”