Description: In the deep ocean, fluxes of particulate organic carbon (POC) and calcium carbonate are positively correlated, suggesting that CaCO3 could increase sinking particle densities and/or protect the organic matter from degradation by prokaryotes, the so called “ballast effect”. Here, we used the PArticle Sinking Simulator (PASS) system to investigate the effect of increasing pressure on the biodegradation of calcifying Emiliania huxleyi aggregates. Incubations were carried out over a period of 10 days, simulating the changes in temperature and pressure in the water column of the NW Mediterranean Sea. Aggregates sinking from a depth of 200 m to 1700 m (assuming an average sinking velocity of 150 m d−1) were exposed to a natural mesopelagic prokaryotic community collected from 200 m. In contrast to previous studies, where silicifying diatom aggregates were used, the calcifying E. huxleyi aggregates were found to be more sensitive to degradation with increasing hydrostatic pressure (relative to constant atmospheric pressure). This was confirmed by changes in lipid composition which suggested increased cell lysis. Changes in particulate inorganic carbon and total alkalinity indicated that CaCO3 dissolution might have been faster under pressure. Increased hydrostatic pressure also had a positive effect on particle aggregation, which may compensate for the effect of increased cell lysis. Our results imply that in coccolithophorid-dominated sinking aggregates, the ballasting and protection effects of coccoliths may collapse throughout the water column. The increased aggregation potential with pressure observed in these controlled conditions, may balance the loss of mineral ballast to a certain extent, although this needs to be confirmed in situ.

Description: Surface and thermocline conditions of the Western Pacific Warm Pool (WPWP) reflect changes in regional and basin scale ocean and atmosphere circulations and in turn may affect climate globally. Previous studies suggest that a range of factors influences the WPWP on different timescales, however the precise forcings and mechanisms are unclear. Combining surface and thermocline records from sediment cores offshore Papua New Guinea we explore the influence of local and remote processes on the WPWP in response to astronomical forcing and changing glacial-interglacial boundary conditions over the past 110 kyr. We find that thermocline temperatures change with variations in Earth's obliquity with higher temperatures coinciding with high obliquity, which is attributed to variations in subduction and advection of the South Pacific Tropical Water. In contrast, rainfall variations associated with meridional migrations of the Intertropical Convergence Zone are primarily driven by changes in insolation due to precession. Records of bulk sedimentary Ti/Ca and foraminiferal Nd/Ca indicate an additional influence of obliquity, which, however, cannot unambiguously be related to changes in precipitation. Finally, our results suggest a thermocline deepening during the Last Glacial Maximum (LGM). A compilation of available proxy records illustrates a dipole-like pattern of LGM thermocline depth anomalies with a shoaling (deepening) in the northern (southern) WPWP. A comparison of the proxy compilation with an ensemble of Paleoclimate Model Intercomparison Project (PMIP) climate model simulations reveals that the spatial pattern of LGM thermocline depth anomalies is mainly attributable to a contraction of the Pacific Walker circulation on its western side.

Description: Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes (εNd) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called ‘concavity index’ (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Fe-oxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the εNd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as ∆εNd Feox-Det) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero ∆εNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1; n = 30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 ± 1.2; n = 44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated ∆εNd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins. Examining the influence of climate and tectonics on measured Nd isotopic compositions, we find that ∆εNd Feox-Det is strongly dependent on temperature in lowlands, following an Arrhenius-like relationship that reflects enhanced alteration of silicate rocks and formation of secondary Fe oxides in warmer climates. In contrast, in high-elevation catchments, ∆εNd Feox-Det defines striking correlation with maximum basin elevation, which we also interpret as reflecting the intensification of silicate weathering and associated Fe oxide formation as elevation decreases, due to the combined effects of thicker soils and warmer temperature. Overall, our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high-elevation environments, while silicate weathering dominates in floodplains. This novel approach combining REE and Nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record, which could be used in future studies to reassess the causal relationships between mountain uplift, erosion and climate throughout Earth's history.