The Northern Gulf of Mexico During OAE2 and the Relationship Between Water Depth and Black Shale Development

Abstract

Despite their name, Oceanic Anoxic Events (OAEs) are not periods of uniform anoxia and black shale deposition in ancient oceans. Shelf environments account for the majority of productivity and organic carbon burial in the modern ocean, and this was likely true in the Cretaceous as well. However, it is unlikely that the mechanisms for such an increase were uniform across all shelf environments. Some, like the northwest margin of Africa, were characterized by strong upwelling, but what might drive enhanced productivity on shelves not geographically suited for upwelling? To address this, we use micropaleontology, carbon isotopes, and sedimentology to present the first record of Oceanic Anoxic Event 2 (OAE2) from the northern Gulf of Mexico shelf. Here OAE2 occurred during the deposition of the well‐oxygenated, inner neritic/lower estuarine Lower Tuscaloosa Sandstone. The overlying organic‐rich oxygen‐poor Marine Tuscaloosa Shale is entirely Turonian in age. We trace organic matter enrichment from the Spinks Core into the deepwater Gulf of Mexico, where wireline log calculations and public geochemical data indicate organic enrichment and anoxia throughout the Cenomanian‐Turonian boundary interval. Redox change and organic matter preservation across the Gulf of Mexico shelf were driven by sea level rise prior to the early Turonian highstand, which caused the advection of nutrient‐rich, oxygen‐poor waters onto the shelf. This results in organic matter mass accumulation rates 1–2 orders of magnitude lower than upwelling sites like the NW African margin, but it likely occurred over a much larger geographic area, suggesting that sea level rise was an important component of the overall increase in carbon burial during OAE2.

Exploring the Impact of Cenomanian Paleogeography and Marine Gateways on Oceanic Oxygen

“…Seven sites document anoxic conditions before OAE2. They are located in the Southern Central Atlantic and in the Gulf of Mexico (Figure 10; Kuypers et al., 2002; Lowery, Cunningham, et al., 2017; Lowery, Leckie, et al., 2017; Owens et al., 2012; Perez‐Infante et al., 1996; van Helmond, Ruvalcaba Baroni, et al., 2014; Westermann et al., 2014). In particular, previous studies suggested that the anoxic area surrounding the Equatorial Atlantic Gateway was driven by the estuarine circulation established between the Pacific and Central Atlantic (Topper et al., 2011; Trabucho Alexandre et al., 2010) but our results demonstrate that this anoxic area is stable even in the ShallowCAS simulation despite the disappearance of the estuarine circulation.…”

Regional chronostratigraphic synthesis of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) interval, Western Interior Basin (USA): New Re-Os chemostratigraphy and 40Ar/39Ar geochronology

“…Moreover, a compilation of global relative sea level records, constrained by ammonite biostratigraphy and/or δ 13 C chemostratigraphy, reveals coeval sequence boundaries and lowstands underlying OAE2 at many localities, including: the English Chalk (Gale et al, 1993), the Paris Basin (Robaszynski et al, 1998), Germany (Voigt et al, 2006;Richardt et al, 2013), Czech Republic (Uličný et al, 1997), Peru (Navarro-Ramirez et al, 2016, Mexico (Elrick et al, 2009), southern New Mexico (Mack et al, 2016), the northern Gulf of Mexico (Lowery et al, 2017), Jordan (Wendler et al, 2014), Lebanon (Grosheny et al, 2017), Israel (Buchbinder et al, 2000), Egypt (Nagm et al, 2014), and Morocco (Kuhnt et al, 2009). Consistency among these global Upper Cretaceous relative sea level records ties the hiatus in the uppermost Hartland Shale in the WIB to an apparent eustatic lowstand preceding the onset of OAE2, coeval with or just preceding a major pulse of LIP volcanism (Fig.…”

Enhanced terrestrial runoff during Oceanic Anoxic Event 2 on the North Carolina Coastal Plain, USA

“…8). Average TOC during OAE2 at Bass River is 1.1 wt % (van Helmond et al, 2014); this is slightly higher than Smith Elementary (0.83 wt %) and Hope Plantation (0.63 wt %) and slightly lower than the next closest published shelf site to the southwest, the Sun Spinks core in Mississippi (1.4 wt %; Lowery et al, 2017). Sequence stratigraphic analysis of Cenomanian-Turonian sediments of the Clubhouse and Bass River formations shows that these sediments represent maximum sea level rise across the boundary on the Atlantic Coastal Plain (Miller et al, 2004;Aleman Gonzalez et al, 2020).…”

“…Estimates of organic carbon mass accumulation rates suggest all three of these inner shelf sites are very similar, ranging from 0.4-0.6 g cm −2 kyr −1 . Average TOC is even higher in the Spinks core (1.4 wt %, or 0.11 g cm −2 kyr −1 ), which represents inner to middle shelf depths during the latter part of OAE2 (Lowery et al, 2017). This suggests the possibility of even higher values on more distal parts of the shelf and highlights the need for a true depth transect (as opposed to four cores from three states) to better understand that variability and better constrain organic carbon burial in this important environment during OAEs.…”