The geological process of diagenesis occurs when soft sediments are turned into hard rocks. Diagenetic features are commonly considered ?secondary features? because they occur after the sediments are buried, mostly isolated from surface conditions. As secondary features, they are usually overlooked when investigating ancient environments. In contrast, PIs recent discoveries suggest that diagenetic processes operating at or just below the seafloor during the time of a mass extinction may indeed hold previously unnoticed clues to the causes of mass extinctions. PIs have discovered unique diagenetic features?calcium carbonate fan layers?associated with the Triassic-Jurassic (T-J) boundary, one of the ?Big 5? mass extinctions, in eastern British Columbia. Textural and geochemical evidence suggests that the calcium carbonate layers grew immediately below the seafloor, likely influenced by ocean chemistry at the time of the mass extinction. Carbon isotopic analysis coupled with mass balance calculations suggest dissolution of preexisting carbonate likely provided the source of calcium carbonate for the early diagenetic layers, potentially indicating ocean acidification in their formation and as a potential extinction mechanism. Here, PIs propose the novel concept that mass extinction mechanisms could affect the diagenetic realm and that studying long-ignored diagenetic features might elucidate the extinction mechanisms in ways not previously exploited. Intriguingly, a literature search turned up unusual carbonate diagenetic features at other T-J boundary sites from around the world, but their nature and occurrence with respect to the extinction is unclear. To determine whether the features are relevant to the extinction, PIs propose a pilot study to investigate unusual diagenetic phases found elsewhere (e.g., St. Audrie?s Bay, England; Lavernock Point, Wales; and Larne, Northern Ireland) within their emerging framework of extinction-related diagenetic effects. The discovery that the carbonate diagenetic features from multiple localities are indeed relevant to the extinction will provide the impetus for a subsequent full-scale study of numerous other T-J sites to exploit a new window?the early diagenetic realm?in the study of mass extinctions.
Mass extinction intervals are associated with biological, geochemical, and sedimentological records that differ from the norm. In this research we demonstrate that the early diagenetic record across the Triassic-Jurassic (T-J) mass extinction, one of the largest mass extinctions in Earth history, may also be exceptional. Intensive studies were made at T-J boundary sections at St. Audrie's Bay (United Kingdom), to complement studies done at Williston Lake (British Columbia, Canada) and New York Canyon (Nevada, USA). Our research team included the two co-I's as well as two women graduate students in the USC Department of Earth Sciences. These geographically disparate T-J boundary intervals host abundant diagenetic carbonate material that likely formed just below the sediment-water interface based upon geochemical investigation, petrographic observations and field relationships. T-J early diagenetic carbonate sometimes manifests as aragonite fan layers that formed in fine-grained siliciclastics just below the sediment-water interface – a facies as yet unknown from other geological intervals. In other cases, very early carbonate concretions are found with unusual morphologies (flat upper surfaces) that potentially reflect a lack of upward accommodation space due to an origin just below the sediment-water interface. All of these phases certainly formed prior to substantial compaction. Another potential manifestation of abundant early diagenetic carbonate is a globally incongruent inorganic δ13C record. Though negative excursions have been recorded across boundary interval sections globally, these excursions differ greatly in both magnitude and absolute value. If early isotopically light early diagenetic carbonate is pervasive, bulk carbonate isotopic signatures may simply reflect the admixture of diagenetic carbonate into the signal rather than an isotopic shift of the global oceanic carbon reservoir. It is uncertain what factors may have contributed to widespread carbonate precipitation in ocean sediments. Abiotic carbonate precipitation on the seafloor is often attributed to elevated carbonate saturation state (e.g. in the Proterozoic or Early Triassic). Curiously, primary carbonate deposits across the T-J boundary are rare. What is certain is that the early diagenetic realm represents a carbon sink the size and variability of which are as yet unknown.
