Global carbon cycle disruption during the latest Pliensbachian (Lower Jurassic) evidenced by simultaneous isotopic depletion in marine and terrestrial carbon pools
Year: 2024
Type: article
Abstract: The Pliensbachian-Toarcian (Pl-To) boundary was marked by an extinction event in marine organisms, and localised ocean anoxia – resulting in the deposition of black shales. Negative isotopic excursions in bulk organic carbon in many of these black shales, are widley belived to indiacte that a global carbon cycle disruption accompanied this extinction event. The Pl-To preceded a period of more intense global carbon cycle disruption that occurred during the Toarcian Oceanic Anoxic Event (T-OAE) by around a million years. It is evident from targeted carbon isotopic analyses of marine and terrestrial organic matter that simultaneous disruptions to the marine and terrestrial carbon cycles occurred within the T-OAE. However, it remains a matter of debate whether the main source of carbon emissions was a climate-sensitive methane reservoir, or volcanic processes. No records of terrestrial carbon cycle disruption exist for the latest Pliensbachian, and so the causal mechanisms behind localised anoxia in this stage of the Lower Jurassic are poorly constrained. We present a record of concomitant isotopic depletion in short and long-chain n-alkanes derived from a thin black shale (the Lower Sulphur Band – LSB) deposited during the latest Pliensbachian (located in Yorkshire, UK), and which is likely to be coeval with the Pl-To. We argue that the presence of simultaneous isotopic depletions in these different compound classes implies the presence of a brief global hyperthermal event in the latest Pliensbachian, with a timescale too short to have been captured by previous analyses of the Yorkshire section. We also show how the studied interval postdates a Hg/TOC spike previously attributed to enhanced volcanic activity in the Karoo-Ferrar Large Igneous Province, implying the relationship between volcanic activity and carbon cycle disruption in the latest Pliensbachian is not direct. We discuss the relevance of our data to the stratigraphic record of the Lower Jurassic sediments of Yorkshire, and suggest that the LSB represents a potential marker horizon for the Pl-To in this region. We further argue that the brief climate disruption that occurred in the latest Pliensbachian made the shallow marine environment vulnerable to the development of more widespread marine anoxia, during the succeeding Toarcian stage.
Source: Global and Planetary Change
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
SDG: Climate action
Open Access status: hybrid
Estimated APC paid: $3,260
OpenAlex ID: W4402567491