Post-glacial climate forcing of surface processes in the Ganges–Brahmaputra river basin and implications for carbon sequestration

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 478 (2017): 89-101, doi:10.1016/j.epsl.2017.08.013.Climate has been proposed to control both the rate of terrestrial silicate weathering and the export rate of associated sediments and terrestrial organic carbon to river-dominated margins – and thus the rate of sequestration of atmospheric CO2 in the coastal ocean – over glacial-interglacial timescales. Focused on the Ganges-Brahmaputra rivers, this study presents records of post-glacial changes in basin-scale Indian summer monsoon intensity and vegetation composition based on stable hydrogen (δD) and carbon (δ13C) isotopic compositions of terrestrial plant wax compounds preserved in the channel-levee system of the Bengal Fan. It then explores the role of these changes in controlling the provenance and degree of chemical weathering of sediments exported by these rivers, and the potential climate feedbacks through organic-carbon burial in the Bengal Fan. An observed 40‰ shift in δD and a 3–4‰ shift in both bulk organic-carbon and plant-wax δ13C values between the late glacial and mid-Holocene, followed by a return to more intermediate values during the late Holocene, correlates well with regional post-glacial paleoclimate records. Sediment provenance proxies (Sr, Nd isotopic compositions) reveal that these changes likely coincided with a subtle focusing of erosion on the southern flank of the Himalayan range during periods of greater monsoon strength and enhanced sediment discharge. However, grain-size-normalized organic-carbon concentrations in the Bengal Fan remained constant through time, despite order-of-magnitude level changes in catchment-scale monsoon precipitation and enhanced chemical weathering (recorded as a gradual increase in K/Si* and detrital carbonate content, and decrease in H2O+/Si*, proxies) throughout the study period. These findings demonstrate a partial decoupling of climate change and silicate weathering during the Holocene and that marine organic-carbon sequestration rates primary reflect rates of physical erosion and sediment export as modulated by climatic changes. Together, these results reveal the magnitude of climate changes within the Ganges-Brahmaputra basin following deglaciation and a closer coupling of monsoon strength with OC burial than with silicate weathering on millennial timescales.This work was supported by the National Science Foundation [grant numbers OCE-1333826 and OCE-1333387]

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 11997, doi:10.1038/s41598-018-30091-8.The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.We acknowledge funding support from the Agouron Institute Postdoctoral Fellowship (K.L.F), the US National Science Foundation (Awards: OCE-1333387 and OCE-13333826), the Investment in Science Fund given primarily by WHOI Trustee and Corporation Members, and the Swiss National Science Foundation (Award: 200020_163162)

Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice-rafted detritus records from Core SO75-26KL, Portuguese Margin

A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic δ18O in TS diagrams in conjunction with equilibrium δc fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (< 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic δ13C by 0.7-1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C starts to decrease 1.5-2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5-3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic δ13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core δ18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 450 (2007): 407-410, doi:10.1038/nature06273.Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales

Sedimentation am Kontinentalhang vor Portugal und Marokko im Spätpleistozän und Holozän

Olive-gray mud covers the continental slope off southern Portugal; at the same time sandy, reddishbrown mud was deposited off Morocco. Biogenie carbonate is a main constituent in both sediment types, off Portugal contributing about 25% and off Morocco more than 50%. The sediment cores from the Portuguese and Moroccan continental slope can be correlated by sedimentological and micropaleontological methods (figs. 10, 15). The climatic variations during the late Pleistocene and Holocene are weil documented by the content of ice-rafted debris (figs. 8, 13) and by faunal compos1t1on of planktonic foraminifera (THIEDE 1971 ). Radiocarbon ages indicate that these variations coincide with variations in the climatic record of the Iberian Peninsula based on pollen analysis (fig. 33). The calculation of sedimentation rates allows an estimate of the amount of terrigenous supply and biogenic carbonate production during the last 15.000 years (table 5). Planctonic organisms produced about 400 gram pro 100 cm2 per 1.000 yrs, while the contribution of benthos is relatively low (2.5 gram). The sedimentation rate of planctonic foraminifera increases from the open ocean to the region of the upper continental slope (fig. 27). This trend, caused by the better food supply near the continental influx and upwelling area, appears both off Portugal and Morocco. The sedimentation rate of benthonic remains decreases by a factor of 100 from the upper continental slope to the deep-sea plain (fig. 28). lt can be shown that this decrease is caused mainly by the reduction of available food with increasing water depth (figs. 29, and 30). In contrast to the biogenic sediment supply, which off Portugal and Morocco is not quantitatively different, the terrigenous supply from the weakly humid Iberian Peninsula is 3-4 times greater than the supply from the semiarid Moroccan mainland (fig. 31 ). This difference influences diagenetic processes. Small manganese concretions and reddish staining iron oxides, which remain stable under the oxygenated conditions in the sediments off Morocco, were replaced by pyrite during the destruction of organic matter in the sediments off Portugal through anaero bic bacteria. During the late Pleistocene a maximum of terrigenous and biogenic sediment (fig. 33) was deposited together with additional, mainly coarse sediment brought in by drifting icebergs. Mediterranean outflow water reduces the sedimentation of silt- and smaller sized particles on the Portuguese continental slope between water depths of 500 and 1.500 m. As a result this sediment is mainly deposited in water depths of 2.000 and 3.000 m (fig. 32). This form of sediment distribution and the high sedimentation rate of silt-sized particles under the southern part of the outflow water (core 8066 B) are characteristic of the time at least since the late Pleistocene. This would indicate that since then the outflow pattern of the Mediterranean water has not changed