Responses Of Vegetation And Sediment Provenance To Indian Monsoon And Human Activities In Middle To Late Holocene,A Study From Godavari Delta,India

The Godavari drainage basin in India is characterized by its zonal distribution of the parent rocks and vegetation.Basalt and C4 plants dominate in the upper reach on the Deccan Plateau,while acid igneous rocks,metamorphic rocks and C3 plants prevail in the middle-lower reach.The basalt produces diagnostic magnetic minerals,but the acid igneous and metamorphic rocks contains little.Organic C/N and 813C can effectively deduce the source of organic matter in the sediments,and the environmental magnetism parameters are very sensitive to the distribution of magnetic minerals.For example,?ARM,can indicate the single domain ferri-magnetic minerals resulting from the weathering and erosion of basalts and and HIRM reflects the antiferromagnetic minerals that are easily produced in the tropical semi-arid weather condition on the Deccan Plateau.Therefore,this study uses the methods of organic elemental chemistry and environmental magnetism to study the changes in organic carbon source and sediment provenance in the Holocene core CY obtained from the Godavari delta.Combining the AMS 14C dating results and the records of Holocene Indian monsoon and the past human activities from previous studies,this study reconstructed the succession of the C3/C4 vegetation and history of soil erosion in the Godavari drainage basin during the middle to late Holocene,and discussed their response to the Indian monsoon changes and human activities.This study linked the vegetation succession and soil erosion to the climate change and human social development,therefore to reconstruct the evolution of the paleoenvironment 'and human-environment relationship in India in the middle to late Holocene.Author believe that this study will help provide background for the response of modern human activities to the global warming,and understand the relationship between the increased population and environment.According to the distribution of organic geochemical elements and magnetic parameters and associated magnetic minerals,the sedimentary records organic carbon and sediment provenances in core CY can be divided into six stages.1)Before 6.0 cal.ka BP,?13C is negative,reflecting a dominant organic carbon source of C3 plants.Values of ?ARm and other magnetic parameters are high and ratios of ?ARM/?lf and ?ARm/SIRM are the highest in the whole core,indicating a predomination of fine-grained ferri-magnetic minerals derived from the Deccan basalt.2)During 6.0-4.9 cal.ka BP,?13C is still negative,reflecting the organic matter is mainly contributed from the C3 plant.Values of ?ARM and other magnetic parameters reduce significantly and are the lowest in the core CY,suggesting an obvious decline in the absolute concentration of magnetic minerals and the reduction in the sediment source from the Deccan Plateau.3)During 4.9-3.2 cal.ka BP,value of ?13C increases,inferring an increase in the contribution of C4 vegetation.Values of ?ARm increase again,indicating that the sediment contribution from the Deccan Plateau increases.In addition,values of 813C and the magnetic parameters decrease rapidly at 4.6 cal.ka BP and 4.0 cal.ka BP,reflecting the fast reccovery of C3 vegetation and the reduction in Deccan basalt source.4)During 3.2-3.1 cal.ka BP,values of ?13C increase significantly,which reflecte the increase in C4 organic matter input to the delta.The magnetic parameters such as?ARM also increase significantly,indicating that the enhancement of the Deccan basalt erosion.5)Duing?3.1-2.1 cal.ka BP,values of 813C are still positive,but values of TOC and TN decrease significantly.Thus deposition of organic carbon in the delta is still mainly contributed by C4 plants,but the vegetation coverage in the drainage basin decreased,resulting in a significant reduction in organic input.Meanwhile,?ARm and other magnetic parameters reach the highest values in the whole hole,indicating the degree of weathering erosion reached its peak.Values of ?13C and TOC decline rapidly while values of magnetic parameters increase rapidly at 3.1-2.8 cal.ka BP and 2.1 cal.ka BP,indicating that the fast recovery of C3 vegetation,but a significant decline in vegetation coverage and enhancement in soil/rock erosion.6)During 2.1-1.3 cal.ka BP,?13C is continuouly positive,values of TOC and TN are still low,and ?ARM and other magnetic parameters maintain high values.Thus C4 vegetation dominated the organic matter source and soil/rock erosion was still strong at this stage.The above changes in organic carbon composition and sediment provenance reflect the effects of Indian monsoon and human activity on the paleoenvironmental evolution of the Godavari drainage basin.The reduction in the sediment contribution from the Deccan plateau at 6 cal.ka BP is suggested owing to the decrease in the precipitation caused by the decline in the Indian monsoon.The increase in the contribution of C4 plants to organic carbon in the delta at-4.9 cal.ka BP also reflects the aridification caused by the monsoon weakeing.However,the increase in sediment source from the Deccan Plateau occurred also at-4.9 cal.ka BP,and there is a further increase at 3.2 cal.ka BP,which contradicts the hypothesis of decline in sediment yield in response to the monsoon weakening.Therefore,author suggested that the human activity in the Deccan Plateau should paly an important role in the increases in sediment source of Deccan basalt.Archaeological discoveries show that Neolithic civilization initiated at about 5,000 years ago and Chalcolithic culture started at about 3200 years ago on the Deccan Plateau.Further,the number of dewelling sites increased significantly at-3200 years ago,which author suggest explain the decline in vegetation coverage and intensification in soil/rock erosion in the Deccan Plateau.This study also reveals the different relationship between human and environment during the Neolithic and Chalcolithic cultures in the Deccan plateau.The monsoon precipitation increased,the C3 plants recovered rapidly,and the soil erosion thus weakened at 4.6 cal.ka BP and 4.0 cal.ka BP during the Neolithic culture,reflecting the limited affect of human on the nature.By contrast,when the monsoon recovered and the C3 vegetation increased during the wet events of the Chalcolithic culture,the vegetation coverage decreased and the soil erosion increased,reflecting the intensified human activity during a favorable climate setting.In addition,suthor suggest that the increase in C4 plant contribution during the middle Holocene is not only the response to the monsoon decline,but may also be linked to the increase in sediment source from the semi-arid Deccann Plateau caused by human activities.