Search

We present dissolved organic carbon (DOC) concentrations, particulate organic matter (POM) composition (δ 13 C, δ 15 N, ∆ 14 C, N/C), and particulate glycerol dialkyl glycerol tetraether (GDGT) distributions from a 34-month time-series near the mouth of the Congo River. An end-member mixing model using δ 13 C and N/C indicates that exported POM is consistently dominated by C 3 rainforest soil sources, with increasing contribution from C 3 vegetation and decreasing contribution from phytoplankton at high discharge. Large C 4 inputs are never observed despite covering ≈ 13% of the catchment. Low and variable ∆ 14 C values during 2011 [annual mean = (− 148 ± 82) ‰], when discharge from left-bank tributaries located in the southern hemisphere reached record lows, likely reflect a bias toward pre-aged POM derived from the Cuvette Congolaise swamp forest. In contrast, ∆ 14 C values were stable near − 50‰ between January and June 2013, when left-bank discharge was highest. We suggest that headwater POM is replaced and/or diluted by C 3 vegetation and pre-aged soils during transit through the Cuvette Congolaise, whereas left-bank tributaries export significantly less pre-aged material. GDGT distributions provide further evidence for seasonal and inter-annual variability in soil provenance. The cyclization of branched tetraethers and the GDGT-0 to crenarchaeol ratio are positively correlated with discharge ( r ≥ 0.70; p -value ≤ 4.3 × 10 − 5 ) due to the incorporation of swamp-forest soils when discharge from right-bank tributaries located in the northern hemisphere is high. Both metrics reach record lows during 2013, supporting our interpretation of increased left-bank contribution at this time. We conclude that hydrologic variability is a major control of POM provenance in the Congo River Basin and that tropical wetlands can be a significant POM source despite their small geographic coverage. [ABSTRACT FROM AUTHOR]

The Indo-Pacific Warm Pool (IPWP) is the largest source of atmospheric heating and moisture on Earth, energizing the global moisture and energy budgets and controlling global ocean-atmosphere circulation. However, the mechanisms driving orbital-scale changes in hydroclimate and proxy records of precipitation isotopic composition remain poorly known. Here, we use the isotope-enabled Transient Climate Evolution (iTRACE) experiment to investigate long-term hydroclimate and precipitation isotope changes in the IPWP during the last deglaciation, and their response to different climate forcings (sea level and ice sheet, greenhouse gases, orbital forcing, and meltwater flux). The simulations suggest land-sea configuration as the main factor driving long-term hydroclimate and precipitation isotope changes. The exposure of NW Australian shelf (before 14 ka) excited Bjerknes feedbacks across the equatorial Indian Ocean (IO), leading to a warm/wet western IO and cool/dry eastern IO dipole pattern, with overall drying and more enriched precipitation isotopic compositions over the Maritime Continent. However, the exposed Sunda Shelf and the main body of Sahul Shelf (before 12 ka) experienced locally dry conditions but more depleted precipitation isotopic compositions. Greenhouse gases and orbital forcing contribute to a weaker hydroclimate dipole pattern that opposes the effects of NW Australian shelf exposure. Different regions within the IPWP have different simulated sensitivities to these forcings. The heterogeneous responses of precipitation to different forcings across the Maritime Continent and location-dependent relationships between precipitation and its isotopic composition result from a variety of regional climatological processes and may explain the heterogeneity of isotopic records of hydroclimate around the IPWP. • Investigated deglacial hydroclimate and water isotope changes in IPWP using iTRACE. • Land-sea configuration drives deglacial trends of IPWP rainfall and its isotopes. • Shelf exposure effect excites a zonal hydroclimate dipole pattern along tropical IO. • GHGs and orbital forcing lead to a weaker and opposite dipole pattern across the IO. [ABSTRACT FROM AUTHOR]

• Dissolved and particulate 87Sr/86Sr vary significantly over time in both rivers. • Dissolved and suspended sediment 87Sr/86Sr values are correlated with R2 > 0.9. • Dissolved 87Sr/86Sr variability is governed by relative tributary contributions. • Dynamic sediment-water interactions possibly modify dissolved 87Sr/86Sr. • Results improve knowledge how river 87Sr/86Sr is linked to marine 87Sr/86Sr record. Variations in the marine 87Sr/86Sr record are largely driven by riverine export of weathering-derived strontium. Among the large river systems, the Ganga and the Brahmaputra deliver a disproportionately large flux of radiogenic Sr (i.e., high 87Sr/86Sr) to the ocean. Understanding the sources and processes that make these river systems such important contributors of radiogenic Sr to seawater has been the focus of many studies; extremely radiogenic silicate and carbonate rocks of the Lesser Himalayan formations impart river water with high 87Sr/86Sr values. Furthermore, dissolved 87Sr/86Sr compositions show large temporal variability in both headwater and floodplain catchments; the magnitude of these fluctuations is higher than is observed in any other major river globally. Here, we present dissolved and depth-specific suspended particulate Sr time-series data from the rivers Ganga and Brahmaputra with weekly resolution during the monsoon and monthly resolution during the non-monsoon seasons of 2010–2011. We find that dissolved 87Sr/86Sr compositions of both rivers show substantial short-term variability with week-to-week changes as large as 0.0134 87Sr/86Sr units. Dissolved 87Sr/86Sr compositions correlate strongly with suspended particulate 87Sr/86Sr compositions at all depths. To understand the short-term variability of dissolved 87Sr/86Sr, we developed a hydro-geochemical mixing model using literature-derived Sr concentration and 87Sr/86Sr values of major tributaries combined with 5-day integrated rainfall as a proxy for unavailable discharge data. Results indicate that short-term changes in dissolved 87Sr/86Sr are largely driven by spatio-temporal rainfall variability across the Ganga and Brahmaputra watersheds. We also observe strong correlations between the dissolved 87Sr/86Sr and the particulate 87Sr/86Sr along the depth-profile in both rivers as well as in other large rivers globally. Hence, we corroborate the findings from Tipper et al. (2021) and propose that in-stream interactions with radiogenic sediments during transport further contribute to dissolved 87Sr/86Sr variability. [ABSTRACT FROM AUTHOR]

• Global compilation of compound-specific n -alkyl lipid 14C data. • Intrinsic and extrinsic processes determine the plant wax 14C composition. • Plant wax mean age primarily controlled by temperature and precipitation. Sedimentary high molecular weight (HMW) n -alkyl lipids derived from the waxes of terrestrial plants are common target compounds in biogeochemical and paleoenvironmental research. These plant waxes derive predominantly from the epicuticular cover of vascular plant leaves and their relative and absolute abundances and stable isotopic composition can be used as proxies to decipher, e.g., continental climate and land-ocean carbon transfer processes. In marine sediments, however, compound-specific radiocarbon analysis has revealed that plant waxes are often not syn -depositional, but instead are substantially 14C-depleted ('pre-aged') upon deposition. This 14C-depletion can be caused by various processes that either promote retention of plant waxes during transport from source to sink such as storage in soils or entrainment in deposition-resuspension loops in rivers and on continental shelves or, alternatively, by processes that add HMW n -alkyl lipids from other sources (e.g., petrogenic inputs). Here, we review the intrinsic and extrinsic processes affecting the sedimentary plant wax 14C composition (ranging from chemical processes to continental-scale environmental conditions), how plant wax 14C compositions translate into mean ages, and which processes control plant wax mean ages in marine sediments. Finally, we use a compilation of available and new compound-specific plant wax 14C data to provide a synthesis and evaluate the major controls on plant wax mean ages in marine sediments at the global scale. [ABSTRACT FROM AUTHOR]

Abstract: This study uses 238U–234U–230Th disequilibria in river sediments in order to constrain the transfer times of sediments in alluvial plains of rivers from Himalaya and the Gangetic plain. From the observed distributions we infer sediment transfer times of about 100 ka in the Gangetic plain for rivers taking their source in the Himalayan chain, and longer transfer of about 160–250 ka for foothill-fed rivers. This difference is probably related to the difference in the sediment transport dynamics of these two types of rivers. [Copyright &y& Elsevier]

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n -alkanoic acids and n -alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon ( 14 C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ 14 C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n -alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Despite numerous studies on Himalayan erosion, it is not known how the very high Himalayan peaks erode. Although valley floors are efficiently eroded by glaciers, the intensity of periglacial processes, which erode the headwalls extending from glacial cirques to crest lines, seems to decrease sharply with altitude 1,2 . This contrast suggests that erosion is muted and much lower than regional rock uplift rates for the highest Himalayan peaks, raising questions about their long-term evolution 3,4 . Here we report geological evidence for a giant rockslide that occurred around 1190 AD in the Annapurna massif (central Nepal), involving a total rock volume of about 23 km 3 . This event collapsed a palaeo-summit, probably culminating above 8,000 m in altitude. Our data suggest that a mode of high-altitude erosion could be mega-rockslides, leading to the sudden reduction of ridge-crest elevation by several hundred metres and ultimately preventing the disproportionate growth of the Himalayan peaks. This erosion mode, associated with steep slopes and high relief, arises from a greater mechanical strength of the peak substratum, probably because of the presence of permafrost at high altitude. Giant rockslides also have implications for landscape evolution and natural hazards: the massive supply of finely crushed sediments can fill valleys more than 150 km farther downstream and overwhelm the sediment load in Himalayan rivers for a century or more., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Abrupt changes in the Atlantic meridional overturning circulation (AMOC) are thought to affect tropical hydroclimate through adjustment of the latitudinal position of the intertropical convergence zone (ITCZ). Heinrich Stadial 1 (HS1) involves the largest AMOC reduction in recent geological time; however, over the tropical Indian Ocean (IO), proxy records suggest zonal anomalies featuring intense, widespread drought in tropical East Africa versus generally wet but heterogeneous conditions in the Maritime Continent. Here, we synthesize proxy data and an isotope-enabled transient deglacial simulation and show that the southward ITCZ shift over the eastern IO during HS1 strengthens IO Walker circulation, triggering an east-west precipitation dipole across the basin. This dipole reverses the zonal precipitation anomalies caused by the exposed Sunda and Sahul shelves due to glacial lower sea level. Our study illustrates how zonal modes of atmosphere-ocean circulation can amplify or reverse global climate anomalies, highlighting their importance for future climate change.

Rationale: Compound-specific oxygen stable isotope analysis has been commercially available for about 20 years but the technique has been infrequently applied and practical operating knowledge is limited. Existing reactor designs and chemistry fail to fully exclude contaminating oxygen, are chromatographically flawed, and are generally fabricated from expensive platinum tubing., Methods: As a new interface between the gas chromatograph and isotope ratio mass spectrometer, we used a nickel tube reactor (containing no platinum wires) in a ceramic sheath with countercurrent helium flushing, operated at 1250°C. We analyzed suites of four long-chain alcohols (LCAs), defining the least and most 18 O-enriched LCAs as internal standards. The other two LCAs, co-injected in varying amounts, were treated as unknowns and amplitude-corrected during data reduction., Results: With this design, signals from non-oxygenated compounds are minimized with a hydrogen trickle, while excessive graphitization is counterproductive. A balance of hydrogen and graphite is required to maintain a proper, stable redox state in the reactor. With low extraneous oxygen, the exact hydrogen flow has little effect on final δ 18 O data quality. Overall average δ 18 O values precisions and errors for the unknowns were better than 0.4 ‰ for injections between 2 and 7 nmol per component., Conclusions: Our actively flushed sheath reduces oxygen infiltration to negligible levels and results in a chromatographic flow devoid of dead or poorly swept volumes. The improved design allows measurement of LCAs with accuracy and precision sufficient to enable investigating real processes, and may also have wider applications to other isotopes., (© 2022 John Wiley & Sons Ltd.)

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon ( 14 C) ages on two groups of molecular tracers of plant-derived carbon-leaf-wax lipids and lignin phenols-from a globally distributed suite of rivers. We find significant negative relationships between the 14 C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14 C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change-induced perturbations of soil OC turnover and stocks., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

The storage of organic carbon in the terrestrial biosphere directly affects atmospheric concentrations of carbon dioxide over a wide range of timescales. Within the terrestrial biosphere, the magnitude of carbon storage can vary in response to environmental perturbations such as changing temperature or hydroclimate 1 , potentially generating feedback on the atmospheric inventory of carbon dioxide. Although temperature controls the storage of soil organic carbon at mid and high latitudes 2,3 , hydroclimate may be the dominant driver of soil carbon persistence in the tropics 4,5 ; however, the sensitivity of tropical soil carbon turnover to large-scale hydroclimate variability remains poorly understood. Here we show that changes in Indian Summer Monsoon rainfall have controlled the residence time of soil carbon in the Ganges-Brahmaputra basin over the past 18,000 years. Comparison of radiocarbon ages of bulk organic carbon and terrestrial higher-plant biomarkers with co-located palaeohydrological records 6 reveals a negative relationship between monsoon rainfall and soil organic carbon stocks on a millennial timescale. Across the deglaciation period, a depletion of basin-wide soil carbon stocks was triggered by increasing rainfall and associated enhanced soil respiration rates. Our results suggest that future hydroclimate changes in tropical regions are likely to accelerate soil carbon destabilization, further increasing atmospheric carbon dioxide concentrations.

The Ganges-Brahmaputra (G-B) River system transports over a billion tons of sediment every year from the Himalayan Mountains to the Bay of Bengal and has built the world's largest active sedimentary deposit, the Bengal Fan. High sedimentation rates drive exceptional organic matter preservation that represents a long-term sink for atmospheric CO 2 While much attention has been paid to organic-rich fine sediments, coarse sediments have generally been overlooked as a locus of organic carbon (OC) burial. However, International Ocean Discovery Program Expedition 354 recently discovered abundant woody debris (millimeter- to centimeter-sized fragments) preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the Bengal Fan (∼8°N, ∼3,700-m water depth) with recovery spanning 19 My. Analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically. In the last 5 My, export included C 4 plants, implying that coarse woody, lowland export continued after C 4 grassland expansion, albeit in reduced amounts. Substantial export of coarse woody debris in the last 1 My included one wood-rich deposit (∼0.05 Ma) that encompassed coniferous wood transported from the headwaters. In coarse layers, we found on average 0.16 weight % OC, which is half the typical biospheric OC content of sediments exported by the modern G-B Rivers. Wood burial estimates are hampered by poor drilling recovery of sands. However, high-magnitude, low-frequency wood export events are shown to be a key mechanism for C burial in turbidites., Competing Interests: The authors declare no competing interest.

The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate 1,2 . The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geological timescales 3 . By removing reduced carbon from Earth's surface, this sequestration process promotes atmospheric oxygen accumulation 2 and carbon dioxide removal 1 . Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds 4,5 and protection resulting from interactions with a mineral matrix 6,7 . Although both mechanisms can operate across a range of environments and timescales, their global relative importance on 1,000-year to 100,000-year timescales remains uncertain 4 . Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments and dissolved organic carbon. We find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bond-strength diversity, consistent with the formation of organo-mineral bonds 8 but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineral-bound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed 7 importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may reveal how and why organic carbon preservation-and thus atmospheric composition and climate-has varied over geological time.