A palaeoecological analysis of late quaternary sapropels from the Mediterranean Ridge

Abstract

A PALAEOECOLOGICAL ANALYSIS OF LATE QUATERNARY SAPROPELS FROM THE MEDITERRANEAN RIDGE

By Darren Barry

The sedimentary sequences of the Eastern Mediterranean Sea are interspersed by blackcoloured, organic rich deposits called sapropels. Ranging in thickness from a few millimeters to as much as tens of centimetres these sedimentary layers have been deposited throughout the late Cenozoic. Despite intensive research the exact environmental conditions that led to the deposition of these sapropels are not yet fully understood. Using planktonic foraminifera as a proxy, this research focuses on the deposition of several sapropels. In addition to the more commonly studied sapropels (S1 and S5) S3 and S6 have also been investigated. The principle aim of this research is to reconstruct the palaeoenvironmental conditions during these depositional events. In addition to using the traditional method of analysing faunal assemblages, particular emphasis will be placed on accessing the test size variation of individual species of planktonic foraminifera. This technique has been applied to other events in Earth’s history but not in relation to sapropel deposition. The research is based on data extracted from ODP core 969A (latitude 33.84N, longitude 24.88E, water depth 2200.3 m). With the exception of S1 (7cm), the thickness of each sapropel (S3 – 28cm; S5 – 28cm; S6 – 41cm) allows each event to be analysed at a sufficiently high resolution. One aspect of this study related to the palaeoenvironmental reconstruction of two cores from open ocean sites. Using micropalaeontological data along with multivariate statistical analysis has allowed detailed analysis of palaeoenvironmental variability since the Last Glacial Maximum (LGM). In addition, estimates of sea surface temperature (SST) shows a gradual climate amelioration from the LGM to present. A unique aspect of this research examined the mean test size data of two shallow and one deep dwelling planktonic foraminiferal species over two sapropel events. Particular attention was paid to both biotic and abiotic factors in the understanding of their effects on species growth. While no obvious increase in mean test size was noted as a direct response to sapropel deposition, variability throughout these timeframes is observed. It is concluded that the mean test size of each species responds differently depending on their SST and nutrient requirements. For the Eemian interglacial sapropel S5 and glacial sapropel S6, a palaeoecological analysis utilising palaeoenvironmental indices reconstructed the water column dynamics and trophic status during these depositional events. Sapropel S5 exhibited considerable variability in response to the position of the Cretan gyre while a two phase depositional event in S6 indicated extreme shoaling of the pycnocline.