The Emergence of Pelagic Carbonate Production by Nannoplankton in The Early Jurassic
Abstract
Coccolithophores are among the main carbonate producers in today's Oceans. They participate to the organic-carbon pump and carbonate counter-pump at the same time, and therefore have a fundamental role in the carbon cycle. In this regard, the first occurrence and subsequent diversification of calcareous nannoplankton and in particular of coccolithophorids (225-65 Mya) likely had a major impact on biogeochemical cycles and the regulation of pCO2 in the atmosphere/hydrosphere. Nevertheless, both causes and consequences of the emergence of calcareous nannoplankton between the Late Triassic and the Early Jurassic remain poorly understood. In order to address the possible relationships between the early diversification of coccolithopororids and the evolution of atmospheric CO2 conditions, we performed a quantitative micropaleontological study in the well-exposed sections outcropping along the west coast of Portugal, spanning the entire Lower Jurassic. The accumulation rates of pelagic carbonates were reconstructed using the absolute abundances and size estimates of the most important pelagic carbonate producers as well as the cyclostratigraphic calibration of the studied successions. Our results show that nannofossil absolute abundance, although fluctuating in the Early Jurassic, increased by discrete steps. The first step (up to 0.2 billion of nannofossils per gram of rock) occurred at the base of the Late Pliensbachian and coincided with the deposition of organic matter-rich sediments. A second, more important increase is recorded at the Pliensbachian/Toarcian transition (-183 Mya), when absolute abundance attained values of 1.1 billion of nannofossils per gram of rock. These abundances are comparable to the record of surface sediments deposited under oligotrophic conditions. The following decrease in coccolithophorids observed during the Early Toarcian oceanic anoxic event is particularly intriguing. The dramatic decrease or loss of biocalcification potential could correspond to the development of naked algae, and may represent a return to conditions that prevailed before the emergence of nannoplankton biocalcification in the Late Triassic. The Early Toarcian biocalcification crisis has been previously related to high pCO2 in the atmosphere/hydrosphere system. Nevertheless, it is still unclear whether biocalcification crisis during the Early Toarcian was a direct consequence of increased pCO2 levels or resulted from high CO2- induced environmental changes such as climate warming, enhanced hydrological cycle or nutrient input to oceanic surface waters.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP53B..07M
- Keywords:
-
- 0404 Anoxic and hypoxic environments (4802;
- 4834);
- 0428 Carbon cycling (4806);
- 4900 PALEOCEANOGRAPHY (0473;
- 3344);
- 4930 Greenhouse gases;
- 4944 Micropaleontology (0459;
- 3030)