Planktonic foraminiferal distribution record productivity cycles: evidence from the Aptian-Albian Piobbico core (central Italy) |
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| Authors: | lsabella Premoli Silva Maurizio Ripepe M Emilia Tornaghi |
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| Institution: | Dipartimento di Scienze della Terra, Universitàdi Milano, via Mangiagalli 34, 20133 Milano, Italy;Dipartimento di Scienze della Terra, Universitàdi Firenze, via G. La Pira 4, Firenze, Italy;AGIP spa, 20097 S. Donato Milnnese, Italy |
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| Abstract: | The Aptian-Albian 'Scisti a Fucoidi' varicoloured pelagic sediments in central Italy, show a 'couplet' alternation of carbonate-rich/carbonate-poor layers, which are interpreted as the sedimentary expression of precession (frequency 19–23 kyr). Carbonate content, chromatic variation, and planktonic foraminiferal abundance were analysed at a 1-cm spacing for a 10-m interval of the Piobbico core, specifically drilled through this formation. Spectral analysis of these parameters shows a prominent signal equated to the c . 100 kyr cycle of orbital eccentricity at a sedimentation rate of 5 mm kyr−1. The coherency of the spectral response of each parameter suggests that a single mechanism controlled the whole sedimentary record. Detailed study of planktonic foraminiferal distribution of the same section at 1-mm scale resolves the Milankovitch frequencies of 41 kyr and 18 to 23 kyr, equated with the obliquity and precessional cycles. But foraminiferal abundance is not in phase with carbonate content, which was largely controlled by calcareous nannofossils, but peaked at intermediate carbonate values. The proposed model for explaining the discrepancy at the precessional level is that foraminifera thrived at intermediate values of the precession index, when the environment was only moderately fertile but stable, while during highs of the precession index, mixing of the water column increased fertility and caused calcareous nannofossil blooms and restriction of planktonic foraminifera to few and tolerant species. The resulting bimodality of foraminiferal abundance per precessional cycle appears to be recorded in the spectrum by peaks at the 11 and 14 kyr levels. Cross correlation of foraminiferal abundances with the calcium carbonate curve over 1–2 Myr intervals produces discrepant results (apparent phase lags) which we attribute to differences in the response to the fundamental eccentricity cycles. |
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