High‐resolution carbon cycle and seawater temperature evolution during the Early Jurassic (Sinemurian‐Early Pliensbachian)

Abstract

The Early Jurassic was marked by a progressive recovery from the end-Triassic mass extinction and punctuated by recurring episodes of anoxia. These changes, associated with fluctuations in carbon isotope composition of marine carbonates, remain incompletely understood. Here we present a highresolution carbon and oxygen isotope record for the Early Jurassic based on well-preserved marine mollusks (belemnites) from Dorset, UK. Our new data show a number of d 13 C excursions, starting with a negative excursion at the Sinemurian-Pliensbachian boundary Event followed by lesser negative excursions showing in the Polymorphous, Jamesoni, and Masseanum-Valdani Subzones. The recognition of the Sinemurian-Pliensbachian boundary Event in this study and elsewhere suggests that observed carbon-isotope trends are likely to represent a supraregional perturbation of the carbon cycle. A prominent positive carbonisotope event is also seen within the Pliensbachian Ibex Zone. This event is also clearly evident in the data from belemnites from Spain. This carbon-isotope excursion is not, however, coincident with inferred peak temperatures. The oxygen isotope and Mg/Ca data allow the determination of a number of pronounced Pliensbachian cool events. From the low point in the Brevispina Subzone, oxygen isotopes become more negative coupled with an increase in Mg/Ca values culminating in an Early Pliensbachian thermal maximum during the Davoei Zone. Taken with existing data, it appears that the Pliensbachian is characterized by two major warmings, first within the Davoei Zone followed by warming beginning in the latest Pliensbachian and peaking in the Early Toarcian.

Orbital pacing and secular evolution of the Early Jurassic carbon cycle

“…Subsequent cooling, associated sea-level fall, and restored water-column mixing have been suggested to have released accumulated light carbon through sediment reworking and oxidative and heterotrophic remobilization, causing the negative CIE in the uppermost margaritatus zone (8,10,26,50,51). Similarly, the influx (upwelling/recycling) of 12 C-rich deep waters associated with a climatic cooling trend has been suggested as an alternative driving mechanism for the Sinemurian-Pliensbachian boundary negative CIE (11).…”

“…Increased drawdown of 12 C-enriched organic matter is, however, generally associated with positive shifts in δ 13 C. Conversely, in Mochras, increased TOC values correspond to negative isotope shifts instead (for example, in the angulata and bucklandi zones), but overall no correlation between TOC and δ 13 C TOC is apparent. Other Lower Jurassic geochemical records show that medium-amplitude shifts are preceded by, rather than concomitant with, increased TOC intervals (11,26). This offset may suggest that the accumulation of organic matter took place elsewhere, outside the Cardigan Bay Basin.…”

“…from Dorset, United Kingdom, throughout the Pliensbachian jamesoni, ibex, and lower davoei zones (11). Furthermore, multiple shifts in δ 13 C TOC are recorded in the upper Pliensbachian kunae and carlottense zones of eastern Oregon in the United States, correlative to the mid-margaritatus to spinatum zones of the northwestern European realm (16).…”

“…Stratigraphically expanded shifts were recorded at the Sinemurian-Pliensbachian boundary (8)(9)(10)(11)(12)(13)(14) and the upper Pliensbachian margaritatus and spinatum zones (10,15,16). Furthermore, multiple stratigraphically less extended short-term δ 13 C shifts of ∼0.5 to 2‰ magnitude have been recognized throughout the Hettangian (17)(18)(19), in the Sinemurian (17,(20)(21)(22)(23)(24), and Pliensbachian (10,11,16,22,(25)(26)(27)(28), where they are recorded as individual shifts or series of shifts within stratigraphically limited sections. Some of these short-term δ 13 C excursions have been shown to represent changes in the supraregional to global carbon cycle, marked by synchronous changes in δ 13 C in marine and terrestrial organic and inorganic substrates and recorded on a wide geographic extent (e.g., refs.…”

The Rapoport effect and the climatic variability hypothesis in Early Jurassic ammonites

“…, b, ; Price et al . ), a sea‐level rise (Hallam ), more humid climatic conditions (Dera et al . ) and a potential incursion of warm water masses from the Tethys Ocean (Dera et al .…”