Monsoon signatures in recent corals from the Laccadive Islands

Abstract

X-radiographs of the coral (Porites sp.) collected from several atolls of Lakshadweep show alternate bands of low and high density, formed in non-monsoon period and monsoon period, respectively. The results reveal annual density variations as well as changes in the growth rates. One probable cause for the higher growth rates in the non-monsoon season could be the nutrient deficient environment. In the monsoon season an increase in the availability of the nutrients and particulate organic matter might have reduced water transparency, thereby curtailing the growth rates. Scanning electron microscopy clearly showed the presence of lithogenic and biogenic material in the monsoonal bands and their absence in the non-monsoonal bands. This indicates that runoff from the islands during the monsoon season may supply detrital material which remains suspended in water column for sometime and subsequently settles down into the coral polyps. The total rare earth (Sigma REE) and light rare earth elements (LREE) in the monsoonal bands are enriched in comparison to the non-monsoonal bands. A completely reverse trend is observed in case of the heavy rare earth elements (HREE). The enrichment of LREE in the high density bands is probably due to greater removal during weathering of the continental crust and the consequent availability for incorporation into the coral skeleton. High resolution analysis of elements like Ca, Mg, Na, K, Si, and Al along the growth axis reveals significant intra-seasonal variations, with an inverse relationship between CA and Mg. The likely cause of this negative relationship and increase in Mg with growth rate seems to be Mg incorporation through co-precipitation. Depletion of Ca and enrichment of Si and Al at few spots suggest the presence of detrital materials