Observatoire Aquitain des Sciences de l'Univers

To date, only one species of the terebellid polychaete genus Thelepus Leuckart, 1849 has been described from Africa: T. pequenianus Augener, 1918. During a 2024 biodiversity survey in Arguin National Park (Mauritania), we discovered a second African species that we describe as Thelepus arguinensis sp. nov. It is characterised by notopodia restricted to only 20–35% of the body, three pairs of branchiae, each with numerous filaments, dorsal branchial gap absent, and thoracic uncini lacking a prow. Phylogenetic analyses of mitochondrial 16S and COI sequences confirm its distinctiveness from all congeners with available data. We also redescribe the polynoid Lepidonotus hupferi Augener, 1918 found sharing the tubes of some specimens of T. arguinensis sp. nov. and provide the first molecular data for the species, while extending its known distribution to Mauritania. This study documents a previously unreported facultative, likely commensalistic, association between the two species, highlighting both the underestimated diversity of West African polychaetes and the limited knowledge on the terebellid–polynoid relationships in the region.

Abstract The Nordic Seas are integral parts of the Atlantic meridional overturning circulation and a key region for the global climate. In this study, we combine observational products, largely derived from the Argo array, to examine the changes affecting ocean conditions in the different regions of the Nordic Seas from 2005 to 2020. Significant warming and deepening of isopycnal surfaces have been found across all regions since 2013, resulting in a volume increase of the surface layer above the σ0 < 27.8 kg m−3 isopycnal. A volume budget over three isopycnal layers is built to disentangle the relative roles played by the atmospheric forcing that results in local water mass transformation and large‐scale advection. In eastern basins, an increase in both the advection of Atlantic water and surface water mass transformation led to an expansion of the surface layer after 2012. In contrast, changes found in the Greenland Sea result primarily from a decrease in transport convergence and changes in the surface forcing that ventilates all layers of the region. Our analysis also highlights that an improved sampling of the region, including toward greater depths and on the East Greenland shelves, is required to reduce uncertainties in the water mass transformation budget of the Nordic Seas and to better understand the mechanisms at play.

Abstract Submarine depressions are common features on the eastern Bahamian seafloor but the genesis of the deepest ones (>4000 m in water depth) is not well understood. We conducted a morphometric analysis and compared them to a worldwide database of rounded depressions, whatever their genesis is. The deep Bahamian depressions are large elongated structures, among the largest on Earth, with a width greater than 1000 m and a depth sometimes greater than 200 m. They extend at the toe of the Blake Bahama Escarpment (BBE), one of the tallest escarpments on Earth. Some of them align parallel to the BBE. Other depressions align along large submarine canyon axes. When aligned along canyon axes, the depressions closest to the canyon head are flanked by a topographic high interpreted as a slope-break deposit, i.e., sediment deposited after flow expansion following a hydraulic jump. Turbidity currents in the carbonate canyon system are not permanent processes, but are rather triggered during sea-level highstands when the carbonate platform is flooded. In addition, some depressions are not located in canyon axes. Consequently, the size and location of the depressions are not likely explained by a simple plunge pool mechanical erosion. Rather, our data suggest that all depressions could be initiated by giant karstic dissolution structures (dolines or sinkholes). Under interpretation, those located in canyon axes are sporadically refreshed by carbonate-laden turbidity currents. The height of the outsized chutes marking the crossing of the BBE by the canyon mouth generates a hydraulic jump allowing sediment deposition toward the bottom of the depression. Large depressions observed at this location in the Bahamas were the result of an initial dissolution phase related to retreat of the BBE and the more recent sediment-laden flow activity. The depressions orientated along canyon axes facilitated the regressive erosion that formed the canyons. At present, the depressions located at canyon mouths act as regular plunge pools.

The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging.

Read the full review for this Faculty Opinions recommended article: Complex biotic interactions mediated by shrubs: Revisiting the stress‐gradient hypothesis and consequences for tree seedling survival.

<p>Understanding past variability and forcing mechanisms of the Asian monsoon system is of key importance for better forecasting its behaviour under future global warming scenarios and how this may affect modern societies and economies. So far, knowledge about long-term monsoon variability in mainland Asia is mainly based on proxy records from Chinese speleothems, primarily recording changes of the East Asian Summer Monsoon (EASM). These records have provided evidence for orbital-scale monsoon variability, driven by Northern Hemisphere summer insolation changes, but also for centennial- to millennial-scale reductions in monsoon precipitation. These so-called Weak Monsoon Intervals (WMIs) occurred synchronously to cold intervals in the North Atlantic realm, e.g. during Heinrich Events, pointing at a close hemisphere-scale climatic teleconnection between the North Atlantic and Asia. However, the exact mechanisms that control short-term monsoon variability are still elusive. Moreover, long-term palaeomonsoon proxy records from the core zone of the Indian Summer Monsoon (ISM) are still relatively scarce compared to those from the EASM realm. To identify possible short-term changes in ISM intensity and reconstruct related hydroclimate and vegetation changes on the Indian subcontinent during the interval ~6–74 ka BP, sediments from IODP Site U1446 in the NW Bay of Bengal have been analysed. This site, being located within the reach of the Mahanadi River, is characterized by high riverine input of terrestrial organic matter and thus ideal for high-resolution analyses of pollen content and the stable hydrogen (δD) and carbon (δ<sup>13</sup>C) isotope composition of <em>n</em>-alkanes from terrestrial plant leaf waxes. Here we present preliminary results of δD and δ<sup>13</sup>C analyses on odd-numbered long-chain <em>n</em>-alkanes (<em>n</em>-C<sub>27</sub> to <em>n</em>-C<sub>33</sub>,) extracted from the IODP Site U1446 sediments. These indicate several reductions in ISM precipitation during the last glacial, which occurred parallel to cold events in the North Atlantic realm, e.g. during Heinrich events H1, H2, H4, H5 and H6. In combination with pollen and alkenone-based (U<sup>K’</sup><sub>37</sub>) sea surface temperature data from the same sediments, we aim at (1) providing a comprehensive and high-resolution reconstruction of past ISM variability and associated vegetation changes on the Indian subcontinent and (2) understanding the trigger mechanisms of centennial- to millennial-scale WMIs, particularly in relation to changes in Indian Ocean oceanography.</p>

Plant-plant interactions are reciprocal and include effects on and response to neighbours. Distinct traits confer competitive effect and response ability, but how specific traits determine effect and response in facilitative interactions has not been studied experimentally. We utilized the model species Arabidopsis thaliana to test for trait dependence of facilitative interactions. Salt-sensitive (sos) mutants or salt-tolerant wild-types were exposed to an experimental salinity gradient with and without intraspecific neighbours and the intensity of plant-plant interactions was measured for three performance variables. We tested whether salt tolerance can predict facilitative effect and response and whether a tradeoff exists between competitive ability and tolerance to stress. Interactions shifted very clearly from negative to positive with increasing stress. Salt-sensitive genotypes were less negatively affected by competition but more dependent on facilitation than were wild-types, indicating a tradeoff between competitive ability and stress tolerance. Surprisingly, sensitive genotypes imposed stronger facilitative effects, despite being much smaller under stress, probably because they retrieved more salt from the soil. Stress tolerance defined facilitative effect and response via distinct mechanisms. We advocate more controlled experiments with model species to advance our understanding of the trait dependence of biotic interactions and their consequences for community organization.© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust. PMID: 30298569 Funding information This work was supported by: China Scholarship Council (CSC), International