Abstract
The Cretaceous greenhouse climate was accompanied by major changes in Earth’s hydrological cycle, but seasonally resolved hydroclimatic reconstructions for this anomalously warm period are rare. We measured the δ18O and CO2 clumped isotope Δ47 of the seasonal growth bands in carbonate shells of the mollusc Villorita cyprinoides (Black Clam) growing in the Cochin estuary, in southern India. These tandem records accurately reconstruct seasonal changes in sea surface temperature (SST) and seawater δ18O, allowing us to document freshwater discharge into the estuary, and make inferences about rainfall amount. The same analytical approach was applied to well-preserved fossil remains of the Cretaceous (Early Maastrichtian) mollusc Phygraea (Phygraea) vesicularis from the nearby Kallankuruchchi Formation in the Cauvery Basin of southern India. The palaeoenvironmental record shows that, unlike present-day India, where summer rainfall predominates, most rainfall in Cretaceous India occurred in winter. During the Early Maastrichtian, the Indian plate was positioned at ~30°S latitude, where present-day rainfall and storm activity is also concentrated in winter. The good match of the Cretaceous climate and present-day climate at ~30°S suggests that the large-scale atmospheric circulation and seasonal hydroclimate patterns were similar to, although probably more intense than, those at present.
Highlights
The Cretaceous greenhouse climate was accompanied by major changes in Earth’s hydrological cycle, but seasonally resolved hydroclimatic reconstructions for this anomalously warm period are rare
In the first part of our study, we show that measurements of δ18O and Δ47 in seasonal growth bands in modern shells of the Black Clam
Studies of the V. cyprinoides from the coastal region of Cochin, Kerala (Fig. 1), show that the growth rate of their shells approaches a minimum of 0.25 mm/month during cooler drier months, when the salinity of estuarine water is at a maximum, and peaks at >1.3 mm/month during warmer wetter months, when freshwater discharge lowers salinity[16,17]
Summary
The Cretaceous greenhouse climate was accompanied by major changes in Earth’s hydrological cycle, but seasonally resolved hydroclimatic reconstructions for this anomalously warm period are rare. We measured the δ18O and CO2 clumped isotope Δ47 of the seasonal growth bands in carbonate shells of the mollusc Villorita cyprinoides (Black Clam) growing in the Cochin estuary, in southern India These tandem records accurately reconstruct seasonal changes in sea surface temperature (SST) and seawater δ18O, allowing us to document freshwater discharge into the estuary, and make inferences about rainfall amount. During the Early Maastrichtian, the Indian plate was positioned at ~30°S latitude, where present-day rainfall and storm activity is concentrated in winter. In present-day India, tropical storms and cyclones are associated with increased atmospheric convection during the summer monsoon This activity brings large amounts of fresh water to the sub-continent and increases river discharge, reducing the salinity of coastal waters. It has been argued, based on the presence of hummocky cross stratification, that the sediments containing the shells were transported from a nearby shelf region and re-deposited as a result of storm activity[10]
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