ALBERT

Description: Copper mineralization at the Nallakonda deposit, copper-lead mineralization at the Dhukonda deposit and lead mineralization at the Bandalamottu deposit constitutes the Agnigundala Sulfide Belt of Paleoproterozoic Cuddapah basin. A sandstone-shale-carbonate association of shallow marine origin hosts the mineralization. Copper sulfide mineralizations in sandstones are stratabound and disseminated type and lead sulfide mineralizations occur as stratabound fracture filling veins and/or replacement veins within dolomite. Systematic mineralogical and sulfur, carbon and oxygen isotope studies of the three deposits indicate that the same ore-fluid that deposited copper at Nallakonda at relatively higher temperature, also deposited copper-lead at Dhukonda and lead at Bandalamottu under progressive cooling during migration through sediments. The ore-fluid was a low temperature (〈200°C), oxidized fluid containing enough metal and sulfate to form sulfide mineralization in the Agnigundala Sulfide Belt. Further, the present study indicates thermochemical reduction of basinal water sulfate to account for the sulfur for sulfide ores. It is envisaged that basal red-bed and evaporite bearing rift-related continental to shallow marine sediments might have acted as the source for the metals in low temperature and oxidized ore-fluid. Rift related faults developed during sedimentation in the basin might have punctured the ore-fluid pool in the lower sedimentary succession and also acted as conduits for their upward migration. The ore-fluid, under progressive cooling, deposited the copper sulfide ores at Nallakonda, copper-lead sulfide ores at Dhukonda and lead sulfide ores at Bandalamottu. The ore bearing horizons have participated in deformations during basin inversion without any recognizable change in mineralization patterns.

Description: The past variability of the South Asian Monsoon is mostly known from records of wind strength over the Arabian Sea while high-resolution paleorecords from regions of strong monsoon precipitation are still lacking. Here, we present records of past monsoon variability obtained from sediment core SK 168/GC-1, which was collected at the Alcock Seamount complex in the Andaman Sea. We utilize the ecological habitats of different planktic foraminiferal species to reconstruct freshwater-induced stratification based on paired Mg/Ca and d18O analyses and to estimate seawater d18O (d18Osw). The difference between surface and thermocline temperatures (delta T) and d18Osw (delta d18Osw) is used to investigate changes in upper ocean stratification. Additionally, Ba/Ca in G. sacculifer tests is used as a direct proxy for riverine runoff and sea surface salinity (SSS) changes related to monsoon precipitation on land. Our delta d18Osw time series reveals that upper ocean salinity stratification did not change significantly throughout the last glacial suggesting little influence of NH insolation changes. The strongest increase in temperature gradients between the mixed layer and the thermocline is recorded for the mid-Holocene and indicate the presence of a significantly shallower thermocline. In line with previous work, the d18Osw and Ba/Ca records demonstrate that monsoon climate during the LGM was characterized by a significantly weaker southwest monsoon circulation and strongly reduced runoff. Based on our data the South Asian Summer Monsoon (SAM) over the Irrawaddyy strengthened gradually after the LGM beginning at ~18 ka. This is some 3 kyrs before an increase of the Ba/Ca record from the Arabian Sea and indicates that South Asian Monsoon climate dynamics are more complex than the simple N-S displacement of the ITCZ as generally described for other regions. Minimum d18Osw values recorded during the mid-Holocene are in phase with Ba/Ca marking a stronger monsoon precipitation, which is consistent with model simulations.