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USING FORAMINIFERA TO DETECT AND MODEL PALAEO-ENVIRONMENTAL CHANGE IN THE BRASSO FORMATION, or “Can I Convince You that Quantitative Micropalaeontology Really is Worthwhile?

Abstract

Foraminifera (forams) are beautiful microscopic organisms that are either benthonic (i.e. they live on the seafloor) or planktonic (floating in the water column). That they fossilise readily makes them useful for determining (a) the ages and (b) the environments in which sedimentary rocks were deposited.
Trinidadian forams were much investigated prior to 1965, but have since been relatively little studied outside of industry. Consequently, recently developed quantitative techniques have rarely been applied here. This has limited abilities to interpret Trinidad's palaeo-environmental history. This talk uses foram assemblages in the Early to Middle Miocene Brasso Formation to demonstrate how applying just a few, easily-calculated quantitative measures can greatly increase our palaeo-environmental knowledge.
The percentage of the total foram community comprising planktonics reveals that the Brasso Formation was deposited during at least two major transgressions and regressions during which water depths ranged between ~30m to >500m. These are reflected in changes in the benthonic foram communities. The distributions of species of Brizalina, coupled with the Benthonic Foraminiferal Oxygenation Index (BFOI), show that in both instances changing relative sea levels for a while brought the seafloor into contact with an oxygen minimum zone (OMZ; a low oxygen layer within the seawater column induced by upwelling of nutrient-rich water). The benthonic communities living in more oxygenated waters above and below the OMZ differed. The information function H shows that benthonic foram diversity was reduced at the OMZ core. The early Middle Miocene upwelling created a tropical refuge for the planktonic foram Globigerina praebulloides, which had otherwise retreated to subpolar latitudes. It is postulated that the transgressions were partly tectonic in origin, conglomerates of the coeval Cunapo Formation marking rejuvenation of a hinterland to the north.

Biography for Dr Brent Wilson FGS

Brent Wilson has since 2003 lectured in micropalaeontology, sedimentology and stratigraphy at the University of the West Indies. He came to the Caribbean region in 1989, however, through the British charity VSO, and for many years taught high school maths on Nevis. During that time he undertook a doctorate using time series analysis to model the population dynamics of foraminifera living on seagrasses around the island. This made him aware that micropalaeontologists must apply quantitative methods if they are to extract as much meaning as possible from their data. He moved to Trinidad in 1998 and worked for five years as an industrial micropalaeontologist before joining the UWI to indulge his research interests in foraminiferal ecology and palaeoecology. Brent is also the author of an international worst-seller, Living on an Arc: A Caribbean Memoir.

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