Abstract
Abstract. Organic carbon (OC) enrichment in sediments deposited during Oceanic Anoxic Events (OAEs) is commonly attributed to elevated productivity and marine anoxia. We find that OC enrichment in the late Cenomanian aged OAE 2 at the Demerara Rise was controlled by the co-occurrence of anoxic bottom water, sufficient productivity to saturate available mineral surfaces, and variable deposition of high surface area detrital smectite clay. Redox indicators show consistently oxygen-depleted conditions, while a strong correlation between OC concentration and sediment mineral surface area (R2 = 0.92) occurs across a range of total organic carbon (TOC) values from 9 to 33%. X-ray diffraction data indicate the intercalation of OC in smectite interlayers, while electron, synchrotron infrared and X-ray microscopy show an intimate association between clay minerals and OC, consistent with preservation of OC as organomineral nanocomposites and aggregates rather than discrete, μm-scale pelagic detritus. Since the consistent ratio between TOC and mineral surface area suggests that excess OC relative to surface area is lost, we propose that it is the varying supply of smectite that best explains variable organic enrichment against a backdrop of continuous anoxia, which is conducive to generally high TOC during OAE 2 at the Demerara Rise. Smectitic clays are unique in their ability to form stable organomineral nanocomposites and aggregates that preserve organic matter, and are common weathering products of continental volcanic deposits. An increased flux of smectite coinciding with high carbon burial is consistent with evidence for widespread volcanism during OAE 2, so that organomineral carbon burial may represent a potential feedback to volcanic degassing of CO2.
Highlights
The geological record is punctuated by intervals of widespread organic carbon (OC) enrichment known as Oceanic Anoxic Events (OAEs) (Arthur and Sageman, 1994; Jenkyns, 2010; Schlanger and Jenkyns, 1976)
Since we present data for the carbonate-free fraction, we can discount the possibility that the correlation between mineral surface area (MSA) and total organic carbon (TOC) is due to changes in the relative abundance of carbonate
Trends in MSA do not lead or lag TOC, as might be expected if changes in these indices were indirectly related through a shared environmental control; the proportional sample-to-sample shifts strongly support a direct mechanistic relationship between OC and MSA, implying preservation of sub-micron-scale OC closely associated with clay mineral surfaces, which provide the bulk of sediment MSA (Keil and Mayer, 2014; Kennedy and Wagner, 2011; Ransom et al, 1998)
Summary
The geological record is punctuated by intervals of widespread organic carbon (OC) enrichment known as Oceanic Anoxic Events (OAEs) (Arthur and Sageman, 1994; Jenkyns, 2010; Schlanger and Jenkyns, 1976). A positive δ13C excursion in bulk organic matter (up to 7 ‰) and carbonates (2–3 ‰) implies that global OC burial increased by 130 % during this period (Arthur et al, 1988; Erbacher et al, 2005), and was accompanied by a selective extinction event that most severely affected benthic organisms and has been attributed to deep-water oxygen depletion (Kaiho and Hasegawa, 1994). Organic enrichment in marine sediments during OAEs shows a complex mixture of influences (Arthur and Sageman, 1994; Kuypers et al, 2004), but is most commonly attributed to bottom-water anoxia and/or increased primary productivity A basin-wide tendency toward OC enrichment is modulated by local continental influences that
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