Search

Land-surface subsidence is a major contributor to land loss in many river deltas. New approaches yielding high-resolution data are needed to parse the relevant driving forces. In 2016, we established a novel “subsidence superstation” ∼2 km from the Mississippi River in coastal Louisiana (USA) to measure compaction in a global reference frame as a function of depth in Holocene sediments and deeper subsidence. The site features three borehole optical fiber strainmeters to obtain continuous records of displacement between ∼1.3 m below the surface and depths of ∼11, 25, and 38 m. These data are complemented by an adjacent station providing hydrologic data and near-surface compaction. We also installed three GPS antennas, one of which is mounted to a rod cemented into the Pleistocene basement. A core from one of the boreholes provides insight into the sediment properties of the entire Holocene succession. Five years of records reveal the compaction rate in the material between 1.3 and 38 m is likely less than 0.25 mm/yr. The GPS records yield a subsidence rate of 2.5 mm/yr regardless of the depth of the anchor, thus corroborating the low compaction rates observed by the strainmeters. The new instrumental records show that current subsidence at this location is governed mostly by deformation of the Pleistocene or underlying strata rather than compaction of Holocene material, with the exception of the uppermost meter. The methodology represents an important new approach to mapping subsidence rates at varying depths, providing insight into the mechanisms governing delta subsidence.

The natural wetlands of coastal Louisiana are experiencing rapid subsidence rates averaging 9± 1mmyr1. Recent measurements based on GPS data and CRMS surface elevation tables (SETs) have shown that most of the subsidence is shallow and occurs in the uppermost 5 meters. Sources of subsidence and the origin of their spatial variability are strongly debated. Here we use CRMS SETs together with historic maps of coastal Louisiana to explore two hypotheses: (i) shallow subsidence is a result of accommodation created by (long-term) deep subsidence processes and self-weight consolidation, and (ii) changes in marsh hydrology (groundwater and surface water flows) have led to a recent increase in shallow subsidence. First, we find that, although self-weight consolidation would result in generally high observed shallow subsidence rates, it does not explain the rates nor the spatial variability of the CRMS SET data. Second, based on historic maps, we find that shallow subsidence rates are significantly higher for CRMS sites where shipping canals have reduced their distance to the marsh edge. This is potentially a result from increased sediment deposition, but CRMS data also show altered groundwater levels near the marsh edge. We find some indication that prolonged periods of low water could have led to increases in effective stresses that explain some of the rapid rates of shallow subsidence observed along Louisiana's coastline.

Sea-level and ice-sheet databases have driven numerous advances in understanding the Earth system. We describe the challenges and offer best strategies that can be adopted to build self-consistent and standardised databases of geological and geochemical information used to archive palaeo-sea-levels and palaeo-ice-sheets. There are three phases in the development of a database: (i) measurement, (ii) interpretation, and (iii) database creation. Measurement should include the objective description of the position and age of a sample, description of associated geological features, and quantification of uncertainties. Interpretation of the sample may have a subjective component, but it should always include uncertainties and alternative or contrasting interpretations, with any exclusion of existing interpretations requiring a full justification. During the creation of a database, an approach based on accessibility, transparency, trust, availability, continuity, completeness, and communication of content (ATTAC3) must be adopted. It is essential to consider the community that creates and benefits from a database. We conclude that funding agencies should not only consider the creation of original data in specific research-question-oriented projects, but also include the possibility of using part of the funding for IT-related and database creation tasks, which are essential to guarantee accessibility and maintenance of the collected data.