Geochemistry and paleoclimate changes in sediments, Northern Arabian Sea

• Main Author: Khan, Athar Ali
• Published: University of Edinburgh 1989
• Subjects: 551.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653357

Four selected piston cores (i.e. CD1715, CD1730, CD1739 and CD1738) in a north-south transect from Charles Darwin Cruise CD18/86 are investigated in this study. The northern Arabian sea is under the influence of both variable northwesterly dust winds and seasonal upwelling. Their characteristic effects on the mineralogy and chemistry of the sediments are analysed both areally and temporally. From the accumulation rates of lithogenic and biogenic elements an attempt has been made to understand the climatic conditions of cold and warm periods of late Pleistocene time. For this purpose, an age model developed based on oxygen isotope and carbonate stratigraphy of a core RC-2761. Carbonate content variations of four cores show that southern cores CD1715 and CD1730 record a sedimentation history of last 250,000 years. Cores CD1738 and CD1739 have been dated to the base of stage 5 (i.e. 120 Kys). These cores were sampled at 10cm intervals and sediments were subjected to mineralogical and geochemical analysis (Si, Al, Fe, Ca, Mg, K, Ti, P, Sr, Cr, Ni, Zn, Cu, Br, I, Zr, Ba, La, Ce, Nd, organic carbon, biogenic silica δ13C, δ15N). Optical examination reveals that sediments contain various proportions of terrigenous and biogenous components. SEM examination of detrital grains shows rounded and pitted quartz grains and subhedral dolomite, indicative of aeolian transport. Biogenic components largely include, planktonic foraminiferids and diatoms. Semi-quantitative mineralogical studies by XRD has shown that quartz, feldspar, chlorite and illite are the major minerals in the sediments. Peak intensity ratios of Qtz/Fld, Qtz/Chl and Qtz/Ill indicate that cores in the north relatively contain more chlorite, and feldspar is low compared to quartz. Cores CD1715 and CD1730 are relatively impoverished in quartz and chlorite but contain more feldspar. The relative change of peak intensity ratios in different climatic stages is not very clear. The geochemistry of major elements Si, Fe and K and their ratios to Al covary with peak ratios of specific minerals and these indicate both a textural control as well as areal and temporal variations in mineralogy of the sediments. Northern cores (i.e. CD1738 and CD1739) are relatively coarser grained as shown by higher Si/Al ratios and higher Fe/Al ratios implying more chlorite. The ratios of Si/Al, K/Al generally show contrasting trends in glacial and interglacial stages. The minor element geochemistry of the sediments shows more complex relationships. The patterns of Ti, Cr and Zr ratios to Al are different. Ti/Al and Zr/Al profiles tend to follow each other, while Cr/Al profiles show inconsistent trends. The Zr/Al ratio is invariable in cores and shows a persistent aeolian input. Cr and Cr/Al ratios are very high and suggest derivation from local ophiolites and transport to the deep sea by pluming or by turbidity currents. The geochemistry of the biogenic constituents include calcite, organic carbon, biogenic silica, Ex Ba (aluminosilicate free). Their causal relationship with metals (i.e. Cu, Ni, Zu) possibly suggests a genetic or diagenetic association. Biomarkers contents are higher in cores CD1715 and CD1730 than in cores CD1738 and CD1739. The changing patterns of biomarkers between cold and warm stages show increased biological production in the Holocene (stage 1), interglacial stages 3, 5 and 7. Isotopic analysis of δ13C and δ15N shows that organic matter is of marine origin throughout the cores. Halogens geochemistry (iodine and bromine) and their ratios to organic carbon indicates an oxic depositional environment. Iodine relative to bromine shows preferential loss as a result of burial diagenesis. Halogens and their ratios to organic carbon in glacial/interglacial stages follow organic carbon contents and this suggests a possible association with biological productivity. However, the higher ratios in cores CD1738 and CD1739 may be a result of high sediment accumulation rates. Sedimentation rates increase from cores CD1715 and CD1730 to cores CD1739 and CD1738. Lithogenic fluxes of quartz, aluminium, dolomite, zirconium and titanium show a marked gradient from north to south. This implies provenance from the north and northwest and aeolian transport by northwesterly winds. This source has been persistent throughout the last 250,000 years. Lithogenic flux values are higher in glacial stages and indicate change in climate. The highest lithogenic fluxes are seen in glacial stage 2, and indicates the greatest aridity during the last 250,000 years climatic history. Biogenic fluxes (calcite, organic carbon, excess Ba and biogenic silica) decrease towards the north. Variations of biogenic fluxes between climatic stages show antipathetic trends to that observed for the lithogenic fluxes. This confirms the idea that distribution of biogenic and lithogenic components in NW Arabian Sea sediments is more controlled by the climatic patterns and the peculiar features associated with them. The Halocene and interglacial periods (3, 5 and 7) are characterised by strong SW monsoon winds and upwelling and higher biogenic fluxes occur during these times. The lower biogenic values during the earliest interglacial stages suggest erosion of the sediment. This is evident from the comparison of productivity record of excess Ba flux from ODP core 722 (2000m water depth) and deep sea cores of this study. At stage boundaries 3/4, 5/6 and 6/7 the ridge core (ODP 722) shows high values of excess Ba flux; low values in basin cores at these boundaries are explained by the physical removal due to bottom currents.