Aridification of the Indian subcontinent during the Holocene : implications for landscape evolution, sedimentation, carbon cycle, and human civilizations

Abstract

The Indian monsoon affects the livelihood of over one billion people. Despite the importance of climate to society, knowledge of long-term monsoon variability is limited. This thesis provides Holocene records of monsoon variability, using sediment cores from river-dominated margins of the Bay of Bengal (off the Godavari River) and the Arabian Sea (off the Indus River). Carbon isotopes of terrestrial plant leaf waxes (6 13 Cwax) preserved in sediment provide integrated and regionally extensive records of flora for both sites. For the Godavari River basin the 61 3 Cwax record shows a gradual increase in aridity-adapted vegetation from ~4,000 until 1,700 years ago followed by the persistence of aridity-adapted plants to the present. The oxygen isotopic composition of planktonic foraminifera from this site indicates drought-prone conditions began as early as -3,000 years BP. The aridity record also allowed examination of relationships between hydroclimate and terrestrial carbon discharge to the ocean. Comparison of radiocarbon measurements of sedimentary plant waxes with planktonic foraminifera reveal increasing age offsets starting -4,000 yrs BP, suggesting that increased aridity slows carbon cycling and/or transport rates. At the second site, a seismic survey of the Indus River subaqueous delta describes the morphology and Holocene sedimentation of the Pakistani shelf and identified suitable coring locations for paleoclimate reconstructions. The 6 3 Cwax record shows a stable arid climate over the dry regions of the Indus plain and a terrestrial biome dominated by C 4 vegetation for the last 6,000 years. As the climate became more arid 4,000 years, sedentary agriculture took hold in central and south India while the urban Harappan civilization collapsed in the already arid Indus basin. This thesis integrates marine and continental records to create regionally extensive paleoenvironmental reconstructions that have implications for landscape evolution, sedimentation, the terrestrial organic carbon cycle, and prehistoric human civilizations in the Indian subcontinent. 3 4A mi Tito y mi Sense, por haberme despertado el interes por la Ciencia. 5Creo muy poco en lo que veo. Y de lo que me cuentan... nada.-Juan Siyago 6 Acknowledgements I am indebted to my Advisors for their unconditional support and generosity with their ideas and their time. Thank you for keeping me afloat when the seas were rough and I had started to drown. I learned that persistence is key and Science is humbling. Through this journey Liviu's tenacity and strong guidance, and Tim's enthusiasm and optimistic outlook delivered me safely to shore.I will like to thank my committee members and Chair for their invaluable contributions to improve this thesis, but most importantly for their continuous support, concern and encouragement through out these years. Delia, for always being my advocate and offering your experience to suggest alternative solutions. Valier, I have learned immensely from you in the lab and our discussions have always been frui...

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

“…On the Godavari slope, the terrigenous fraction dominates the total organic carbon (TOC) in marine sediments . In agreement with this, TOC radiocarbon ages in our core have been previously found to be remarkably similar to co-located ages of the strictly terrigenous higher plant leaf-wax fraction (Ponton, 2012). This age similarity also excludes interferences from within-river biological productivity (e.g., Eglinton and Hamilton, 1967;Eglinton and Eglinton, 2008).…”

“…2). The overall trend of the δ 13 C leaf-wax record agrees with the view that the seasonality of Northern Hemisphere insolation (Ponton et al, 2012) led to progressively weaker monsoons over the Holocene. However, two clear aridification steps are evident: between ∼ 5000 and 4500 years ago, and ∼ 1700 years ago (Fig.…”

Response to interactive comment on “SHORT COMMUNICATION: Massive Erosion in Monsoonal Central India Linked to Late Holocene Landcover Degradation” by Liviu Giosan et al. by Y. Kulkarni

“…Offshore from the Godavari mouth, a persistent sediment plume extends over 300 km during the monsoon season, when over 90 % of the fluvial sed-iment is discharged (Sridhar et al, 2008). Because the shelf in front of the delta is unusually narrow (i.e., under 10 km at our core location) copious sediment deposition takes place directly on the continental slope, resulting in sediment accumulation rates as high as 250 cm kyr −1 (Ponton et al, 2012). Owing to the narrow shelf, changes in sea level would also have minimal effects on sediment deposition at our site, especially after the early Holocene when the global sea level reached within a few meters of modern values (Lambeck et al, 2014).…”

“…1) in the Bay of Bengal (NGHP-01-16A at 16 • 35.6 N, 82 • 41.0 E; 1268 m water depth; Collett et al, 2015). The age model for the core based on 11 radiocarbon dates on mixed planktonic foraminifera was previously published by Ponton et al (2012). The Godavari Basin was not affected by tectonics on the Holocene timescale or by glacial/snow meltwater and strong orographic precipitation, which augment and complicate the water and sediment discharge of the larger Himalayan rivers like the Ganges or Brahmaputra.…”

“…The Godavari Basin was not affected by tectonics on the Holocene timescale or by glacial/snow meltwater and strong orographic precipitation, which augment and complicate the water and sediment discharge of the larger Himalayan rivers like the Ganges or Brahmaputra. Instead, it integrates rainfall from the Core Monsoon Zone (CMZ), the region of central India that is representative of both the mean monsoon regime and its fluctuations over the peninsula (see Ponton et al, 2012, and references therein). Consequently, over 90 % of the Godavari's water discharge into the Bay of Bengal derives from summer monsoon precipitation (Rao et al, 2005), making its sedimentary deposits a prime target for continental climate reconstructions and a repository for sedimentary proxies of erosion prior and after the Neolithic adoption of agriculture in central India.…”