Comparing land-based and deep-sea rock and fossil records of microplankton to test for bias in diversity patterns through time

Lead Research Organisation: The Natural History Museum
Department Name: Earth Sciences

Abstract

Recent reviews of how much marine sedimentary rock remains preserved on land have highlighted just how variable that record is from different time periods, both in terms of environmental representation and absolute rock volume. As a result we now know that the rock record over the continental shelf is very cyclical in structure. At its most fundamental level the Phanerozoic rock record comprises two cycles defined primarily by changes in the ratio of marine to terrestrial rock. These cycles are evident across both Northern and Southern hemispheres although their precise timing differs slightly. They arise in response to major global tectonic cycles of plate assembly and rifting. Superimposed on these are shorter-term (ca. 50 My) cycles, which are largely independent in timing from continent to continent and which reflect regional tectonic events and local sea level changes. Marine biodiversity, as measured by counts of macrofossils, also shows a cyclical pattern. These cycles match changes in the extent of continental flooding and thus could be generated through one of two processes: they could arise because of sampling bias, as the amount of marine sedimentary rock of a particular age that is ultimately preserved varies as seas expand and contract over the continents; or biological diversity could be rising and falling in tandem with the expansion and contraction of shallow marine seaways. At present it is not clear which is the predominant driver of the marine fossil record because there is high spatial heterogeneity of faunas within shallow water environments leading to a very strong species-area effect (the wider an area sampled the more species will be found). So far all the attention has been focussed on the rock and fossil records of the continental shelves. Large scale analyses of the nature of the deep-sea record and whether it correlates with marine diversity have never been explored. There are strong grounds for thinking that the deep-sea rock and fossil records might be different from that preserved on the continental shelves. For one thing much of the sedimentary record comprises near continuous biogenic deposits derived from planktonic microorganims which only rarely encroaches far onto the continental shelves. Furthermore, in the open oceans marine plankton are widely dispersed, so that any species-area effect is much less evident. Yet we also know there are hiatuses and barren intervals in the deep sea due to non-preservation, and the numbers of cores yielding fossiliferous rocks of a specific age is unlikely to be uniform over time. What is more, planktonic foraminiferans are reported to show a 30 myr cyclical rise and fall in diversity. To clarify whether fossil diversity patterns are causally linked to rock sampling patterns we will focus on the fossil record of two important planktonic groups, coccolithophorids and planktonic foraminifera. We will first identify any possible biases in the deep-sea record by assessing the distribution of gaps and non-preservation hiatuses in the sampled record of the central Atlantic, and test how much these actually affect diversity counts (for geographically widespread taxa variation in density of sampling should have a minimal effect). By recording their diversity through time from deep-sea cores and comparing this with their diversity patterns as derived from land-based outcrops, we hope to be able to show whether the 30-Myr cyclicity is a genuine biological signal or an outcome of the extent to which open oceanic sedimentation encroaches onto the continental blocks during high sea-level stands. We will also compare the nature of the deep-sea sedimentary rock and fossil records with rock and fossil records derived from land-based sections on adjacent continents to find out whether the deep-sea fossil record is immune from many of the biases affecting the shallow water record, or whether both covary in quality.

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