AN EARLY DIAGENETIC CARBONATE FACTORY ACROSS THE TRIASSIC-JURASSIC TRANSITION

Geographically disparate Triassic-Jurassic (T-J) boundary successions host abundant diagenetic carbonate material. Geochemical investigation, petrographic observations and field relationships suggest the diagenetic carbonates likely formed just below the sediment-water interface prior to compaction. 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 potentially unknown from other geological intervals. In other cases, very early carbonate concretions are found with unusual morphologies (flat upper surfaces) that may reflect a lack of upward accommodation space during growth just below the sediment-water interface. All of these phases certainly formed prior to substantial compaction. The globally incongruent inorganic δ¹³C record across the T-J boundary may also reflect the preponderance of early diagenetic carbonate. Though negative excursions have been recorded across boundary interval sections globally, excursions differ greatly in both magnitude and absolute value from locality to locality. If ¹³C depleted early diagenetic carbonate is pervasive, bulk carbonate isotopic signatures may simply reflect the admixture of diagenetic carbonate 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 uncommon. What is certain is that the early diagenetic realm represents a carbon sink the size and variability of which are as yet unknown.