Read Full Article (file size: 2668234 bytes)    Cited by

PALEOCEANOGRAPHY, VOL. 19, PA1008, doi:10.1029/2003PA000884, 2004

Nannofossil carbonate fluxes during the Early Cretaceous: Phytoplankton response to nutrification episodes, atmospheric CO₂, and anoxia

Elisabetta Erba

Dipartimento di Scienze Della Terra “A. Desio,” Milan, Italy

Fabrizio Tremolada

Dipartimento di Scienze Della Terra “A. Desio,” Milan, Italy

Abstract

Greenhouse episodes during the Valanginian and Aptian correlate with major perturbations in the C cycle and in marine ecosystems, carbonate crises, and widespread deposition of Corg-rich black shales. Quantitative analyses of nannofossil micrite were conducted on continuous pelagic sections from the Southern Alps (northern Italy), where high-resolution integrated stratigraphy allows precise dating of Early Cretaceous geological events. Rock-forming calcareous nannofloras were quantified in smear slides and thin sections to obtain relative and absolute abundances and paleofluxes that are interpreted as the response of calcareous phytoplankton to global changes in the ocean-atmosphere system. Increased rates of volcanism during the formation of Ontong Java and Manihiki Plateaus and the Paranà-Etendeka large igneous province (LIP) are proposed to have caused the geological responses associated with early Aptian oceanic anoxic event (OAE) 1a and the Valanginian event, respectively. Calcareous nannofloras reacted to the new conditions of higher pCO₂ and fertility by drastically reducing calcification. The Valanginian event is marked by a 65% reduction in nannofossil paleofluxes that would correspond to a 2–3 times increase in pCO₂ during formation of the Paranà-Endenteka LIP. A 90% reduction in nannofossil paleofluxes, which occurred in a 1.5 myr-long interval leading into OAE1a, is interpreted as the result of a 3–6 times increase in pCO₂ produced by emplacement of the giant Ontong Java and Manihiki Plateaus. High pCO₂ was balanced back by an accelerated biological pump during the Valanginian episode, but not during OAE1a, suggesting persisting high levels of pCO₂ in the late Aptian and/or the inability of calcareous phytoplankton to absorb excess pCO₂ above threshold values.

Received 23 January 2003; accepted 31 October 2003; published 10 February 2004.

Index Terms: 1040 Geochemistry: Isotopic composition/chemistry; 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; 1615 Global Change: Biogeochemical processes (4805); 1635 Global Change: Oceans (4203).

Read Full Article (file size: 2668234 bytes)    Cited by

Citation: Erba, E., and F. Tremolada (2004), Nannofossil carbonate fluxes during the Early Cretaceous: Phytoplankton response to nutrification episodes, atmospheric CO₂, and anoxia, Paleoceanography, 19, PA1008, doi:10.1029/2003PA000884.

Copyright 2004 by the American Geophysical Union.