59th Annual Report on Research 2014

Coniacian-Santonian Oceanic Anoxic Event 3 (OAE3) is the last and least studied of the Mesozoic anoxic events. It is mainly known from the central Atlantic, Caribbean region, and the US western interior, where it is associated with an abrupt increase in total organic carbon (TOC) and corresponding trace metal indicators for anoxia. However, the nature of redox conditions before this major shift is largely unknown. Were anoxic conditions confined to the sediments, and how was life on the seafloor and in the water column affected by anoxia? Foraminifera, marine protists with a readily preservable shell, or test, are often used to ascertain changes in redox conditions, and the presence or absence of taxa that occupy various ecological niches can yield information about where in the water column or sediments these changes occurred.

The Upper Cretaceous Niobrara Formation (Ft. Hays Limestone and Smoky Hill members) of the US western interior has been the target of oil and gas drilling for decades. Although the Niobrara was deposited during a time of enhanced organic matter (OM) production and preservation associated with Oceanic Anoxic Event 3, understanding local variations in oceanographic conditions can help explain and predict variations in OM content across the Western Interior Basin. Here we present planktic and benthic foraminifer paleoecological data spanning the onset and duration of OAE3 in the central part of the Western Interior Seaway (WIS) from two cores (Portland, Angus) near the Colorado Front Range and a classic series of Niobrara outcrops in western Kansas to determine: 1) if there is any evidence for changes in dissolved oxygen prior to the onset of OAE3; 2) whether anoxia was limited to bottom waters or whether it intruded into the upper water column; and 3) the biotic response to these changes.

Foraminiferal paleoecology in the Fort Hays Limestone Member suggests a likely response to relative sea level change; higher percent benthics and low keeled diversity in the lower Fort Hays give way to lower percent benthics and higher keeled diversity toward the top of the member, which records the highstand of the Niobrara Cycle. In the lowermost Smoky Hill Shale Member, percent benthic values continue to decrease. Benthic foraminiferal abundances of ~5-15 percent are far too low for a shallow seaway that was ~100-150 m deep during peak transgression, and likely shallower. In addition to possible dissolved oxygen influences on benthic foraminiferal abundances, food supply is also a major control on benthic assemblages. However, organic carbon was preserved in the sediments, suggesting that food was not a limiting factor on benthic life. Changing salinity could also affect benthic assemblages, especially if riverine runoff induced a stable stratification of the water column, which in turn could have reduced seafloor oxygenation. However, reduced salinity would also directly affect planktic assemblages. The fact that planktic forams remain relatively constant, and their small-scale fluctuations don't track benthic abundances, suggests that salinity stratification was not the primary reason for low benthic foraminiferal abundances.

This leaves changes in dissolved oxygen as the most likely primary environmental stress on benthic foraminifera in the Smoky Hill Shale. The extremely depressed and declining percent benthic values in the lower shale and limestone at all three sites suggest a decline in benthic oxygen concentrations before the onset of anoxia as indicated by trace metal markers and elevated TOC values at the base of the lower shale unit of the Smoky Hill. Following a brief recovery in the upper part of the lower shale and into the lower limestone, benthic and keeled planktic foraminifera decrease, with occasional brief, one-off repopulation events – oxygenation events. This steady state persists through the top of the Smoky Hill Chalk in the Kansas outcrops, equivalent to ~3 myr, during which time TOC remains elevated and almost no benthic foraminifera live in the relatively deep central axis of the Niobrara Sea.

A fluctuating strandline along the western edge of the seaway during Coniacian-Santonian time suggests that relative sea level changes may be responsible for fluctuating carbonate content of the Smoky Hill Member, but this was not the principle driver of anoxia at the seafloor. Increased riverine runoff and nutrient input during the regression of the Niobrara Sea likely drove marine productivity and excess organic matter flux to the seafloor creating oxygen stress to the benthos and anoxic to euxinic conditions, which were exacerbated by increasing restriction of the seaway as long-term sea level fell. This is particularly true in the central and eastern parts of the seaway, which were bathed in warm, oxygen-poor Tethyan water. Sites on the western margin of the Niobrara Sea saw little change in benthic populations, as cool, well-oxygenated Boreal waters and shallower water depths kept the western margin more habitable for benthic life. The mechanisms of hypoxia development in shallow seas have major implications for restricted environments such as the WIS, as well as continental shelves, a topic as relevant to future change as it is to the past.

CONCLUSIONS

1. A long (~3 myr) period of organic-rich shale deposition and bottom-water anoxia, as evidenced by a lack of benthic foraminifera and concurrent changes in redox sensitive trace metals, characterizes much of the Smoky Hill Shale Member of the Niobrara Fm. This interval corresponds to Oceanic Anoxic Event 3.

2. Prior to the 'onset' of OAE3, defined by an abrupt increase in TOC, abnormally low abundance of benthic foraminifera suggests a gradual decrease in dissolved oxygen following peak transgression of the Niobrara Sea.

3. A rapid shift to anoxia in the lower Smoky Hill Shale was preceded by a slow decline in benthic dissolved oxygen suggesting that a threshold condition drove the depositional system of this restricted basin into a new steady state.

4. Anoxia and TOC enrichment in the middle part of the seaway was made possible by increased riverine nutrient flux, the presence of warm, low-oxygen Tethyan waters in the central and eastern parts of the seaway, and the overall restricted nature of the WIS during the regressive phase of the Niobrara cycle.