Reliability of Molluscan Death Assemblage in Capturing Depositional Processes: Insights from a Tropical Shallow Marine Setting

Sankar S, Ammu (2017) Reliability of Molluscan Death Assemblage in Capturing Depositional Processes: Insights from a Tropical Shallow Marine Setting. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Ammu Sankar. S (12MS025)) Ammu_Sankar,_12MS025.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

An important aspect in the paleoecological studies is the extent to which a fossil assemblage represents a biological community. Biological communities are considered to be made up of individuals that live within a measurable physical limit of habitats. Fossil assemblage often represents only a portion of the original living community due to preservational bias. On the other hand, a time-averaged assemblage has a higher probability of capturing the species diversity that is often lost in ecological sampling due to patchiness of distribution. Comparison between live and death assemblage has often been used to evaluate the effects of time averaging and taphonomy on the species diversity and distribution. The main aim of this study is to understand the reliability of molluscan death assemblage in capturing the depositional processes in a shallow marine setting. If the death-assemblage is not a random subsample of live assemblage, then it ought to indicate taphonomic processes that resulted in such non-random sorting. Using the molluscan death and live assemblage of five depositional environments (beach, tidal flat, estuary, restricted part, sand bar) from the shallow marine setting of Chandipur-on-sea, we evaluated the reliability of species distribution in predicting depositional environment. Because grain size is one of the strongest proxies for energy of transportation, we evaluated the change in size distribution of dead shells in different environments considering them as sedimentary particles. Our model demonstrated that the environments with highest variation in size between live and death assemblages correspond to higher energy; while the low-energy environments such as estuary, restricted area and behind sand bar portions has low variation in size between the live and death assemblage. Our study shows the effect of transportation in shaping the time-averaged accumulation; this emphasizes the usefulness of transported deathassemblages that are non representative of the live community in predicting the depositional process.

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