Moss Landing 海洋实验室

The description of the bed topography under the Greenland and Antarctic Ice Sheets has greatly improved over the past decade through new field campaigns and mapping techniques, leading to BedMachine, a high-resolution gridded bed map widely used by the ice-sheet modelling community. Despite regular updates, BedMachine still suffers from uncertainty in ocean bathymetry and mapping artefacts in the ice-sheet interior. We describe here four recent improvements that address these limitations. In Greenland, we use ICESat-2 surface elevation time series to construct an ensemble of bed elevations that captures finer bed details. In Antarctica, we use Ice-Flow Perturbation Analysis in the interior. This approach provides an estimate of the bed topography using the surface expression of mesoscale bedforms. For periphery ice caps and the Antarctic Peninsula, we use the machine learning-based IceBoost approach, which is capable of inferring fine details based on surface features. Finally, over the continental shelf, we use a new gravity inversion product from the Antarctic Gravity Anomaly Grid, which provides significant refinements to the bathymetry around the entire ice sheet. Overall, these represent major improvements in the description of the bed topography and bathymetry for both ice sheets. This article is part of the Theo Murphy meeting issue 'Next generation ice-sheet bed measurements'.

Phytoplankton are critical to the Antarctic marine food web and associated biological carbon pump, yet long-term shifts in their community composition are poorly understood. Here, using a machine learning framework and combining pigment samples and environmental samples from austral summertime 1997-2023, we show declines in diatoms and increases in haptophytes and cryptophytes across much of Antarctica's continental shelf. These trends-which are linked to sea ice increases-reversed after 2016, with a rebound in diatoms and a large increase in cryptophytes, coinciding with the loss of sea ice. Significant changes (P < 0.05) across the 25-year dataset include diatom chlorophyll a (chl-a) declines of 0.32 mg chl-a m(-3) (similar to 33% of the climatology) and increases for haptophytes and cryptophytes of 0.08 and 0.23 mg chl-a m(-3), respectively. The long-term shifts in phytoplankton assemblages could reduce the dominance of the krill-centric food web and diminish the biologically mediated export of carbon to depth, with implications for the global-ocean carbon sink.

DNA metabarcoding of plankton samples is a cost-effective approach to analyze the richness and composition of zooplankton in coastal waters. Plankton surveys using metabarcoding can also be used to monitor species of concern, both for planktonic species and benthic species with planktonic larvae. We used DNA metabarcoding with the mitochondrial cytochrome oxidase I gene to: (1) evaluate time of sampling (year, month, week), tidal cycle, time of day, and sampling location as potential sources of variation for zooplankton community richness and composition, and (2) as an early detection tool for species of concern in Prince William Sound. We found that seasonality of sampling had the strongest impact on species richness and community composition. Differences in community composition were mostly driven by differences among meroplankton groups, with the peak in meroplankton abundance occurring in April and May, depending on the location. We detected eight potentially introduced species, three holoplanktonic (Limnoithona tetraspina, Oithona davisae, Pseudodiaptomus marinus), one benthic (Monocorophium acherusicum), and four meroplanktonic species (Amphibalanus improvisus, Philine auriformis, Alitta succinea, Marenzelleria neglecta). Most of these potentially introduced species detections occurred in a single sample with low read counts; but Oithona davisae was detected in three locations over two years. We discuss confidence in the genetic ID and invasion history for each of the species of concern. We showed that (1) plankton surveys can be a broad monitoring tool for species of concern, (2) timing of sampling can be critical depending on the organisms' life histories, and (3) sampling during the meroplankton peak concentration can increase the chance of detecting larval stages of introduced benthic species. Finally, we provide some bioinformatic recommendations to improve species detection and validate identifications.

Although prevalent for the late Holocene, relative sea level (RSL) constraints during and immediately after the Last Glacial Maximum (LGM) are sparse. This scarcity of data is particularly pronounced along mid-latitude shelves such as central California, which lack post LGM RSL constraints older than 12 ka. In this study we collected 7 sediment cores and high-resolution seismic data from Estero Bay to constrain RSLs across the central California shelf between similar to 9 and similar to 16 ka. We reconstructed these RSLs using two sea-level indicators found within our sediment cores: the wave ravinement shell hash burial surface (WRSHBS) and the sedimentary contact between offshore mud facies and ripple cross-laminated sands. To determine the indicative meaning of these two sea-level indicators, we examined the relationship between the local wave regime, modern bathymetric profiles, and the depth of preservation of each sea-level indicator. After correcting for tectonic uplift, we estimated sea levels in central California to have been similar to 39 +/- 7.5 and 49 +/- 7.5 m below present sea level between 9 and 12 ka, in agreement with previous RSL reconstructions along this coast. Between 13.8 and 15.9 ka, we estimate sea levels to have reached similar to 86 +/- 8-99 +/- 8 m below present sea level. Our findings offer a Late Pleistocene RSL reconstruction for central California and develop new methodologies for estimating past RSLs on similar mid-latitude shelves.

As part of the PANDORA cruise (GEOTRACES GP12), concentrations of dissolved iron (dFe) were measured at 11 stations inside and outside the Solomon Sea, a semi-enclosed sea in the western tropical Pacific, with complex topography and straits, and strong western boundary currents supplying the equatorial current system. These measurements aimed to better assess the various sources of dFe in our study area and the Solomon Sea's potential as a source of dFe for the Equatorial Undercurrent (EUC). A simple box model allows calculating and discussing the fate of the dFe in the different water layers flowing through the Solomon Sea and suggests that the amount of dFe enrichment within the enclosed sea was not significant for the lower thermocline and intermediate waters, indicating that most of the dFe was acquired prior to reaching the Solomon Sea at the entrance and/or that inputs are approximately balanced by scavenging within the basin for these two layers. In contrast, dFe enrichment was significant for the upper thermocline layer and the deep waters, highlighting enrichments from external sources, as well as combination of internal processes, such as scavenging and/or organic complexation. The relative dFe contribution of the Solomon Sea to EUC was 20 %, on average. Other sources might thus provide dFe to the EUC, along the water transport downstream of the Solomon Sea (e.g. Bismarck Sea) or from the northern hemisphere. Diazotrophs such as Trichodesmium might also contribute to the dFe enrichment of the EUC after export and remineralization at depth outside the Solomon Sea.