Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

Soft rock cliff retreat under changing climate drivers

<p>Shoreline retreat can happen rapidly in cliffs composed of loosely consolidated glacial and pre-glacial sediments. Typical centennial-scale average retreat rates for some cliffed coastlines of East Anglia, UK are 2 - 5 m a<sup>-1</sup> where cliffs have no protection from storm energetics. Recent research using pre- and post-storm clifftop (geomorphological) surveys, as well as aerial photographs, has shown that in single events retreat can be 3 – 4 times the long-term average, with up to 15 m of retreat in a single event. Periods of clifftop stasis are thus interspersed with short term shocks, when meteorological conditions generate energetic drivers of change (elevated still water levels, high onshore waves, high rainfall inputs). Furthermore, short term shocks deliver enhanced sediment supply to the nearshore region, which is an important factor to take into account within future management planning strategies.  </p><p>This paper uses the latest Earth Observation data to quantify and evaluate rates of soft rock cliff retreat, thereby identifying periods when short term shocks have been delivered to the cliffs. It then explores the climate drivers of these shocks and assesses the associated synoptic meteorological scenarios. Finally it considers the implications for quantities of sediment released, in terms of both overall magnitude and alongshore variability.</p><p>Results suggest that three recent events stand out as having a significant impact on rates of cliff retreat and associated sediment release on the Suffolk coast, southern North Sea. The “Big Freeze” of the UK winter of 2010-11 involved a protracted period of easterly air flow from mid-November and into December, 2010. The process drivers were high magnitude onshore winds, generating nearshore waves of around 4 m. The 5 December 2013 North Sea surge similarly resulted in rapid cliff retreat and sediment release. In this event winds were alongshore so the wave impacts were lower, but the elevated water levels generated by the surge meant that wave action could be directed onto the cliffs. By far the biggest recent event in terms of storm forcing energetics was the February – March 2018 “Beast from the East” and “Mini Beast”, where persistent onshore winds generated waves of almost 4.5 m at Southwold Approaches (highest on record) that coincided with two phases of high spring tides (no surge). When regional-scale Sudden Stratospheric Warming (SSW) generates strong and persistent easterly winds there are widespread potentially irreversible consequences for cliff and beach sediments around the western North Sea coastline.</p>

Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterized by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilized by microbial communities. The degradation of POM is associated with dissolved organic matter (DOM) production and reworking. The release of recalcitrant DOM to the overlying waters may represent an important organic matter escape mechanism from remineralization within sediments but received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion–driven fluxes and net in situ fluxes of DOC and DON in order to investigate processes affecting DOM cycling at the sediment–water interface along a transect 12° S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore-water DOC and DON accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralization within sediments. High DON accumulation resulted in very low pore water DOC / DON ratios (> 1) which could be caused by either an imbalance in DOC and DON remineralization, or to the presence of an additional nitrogen source. Diffusion driven fluxes of DOC and DON exhibited high spatial variability. They varied from 0.2–0.1 mmol m−2 d−1 to 2.52–1.3 mmol m−2 d−1 and from −0.042–0.02 mmol m−2 d−1 to 3.32–1.7 mmol m−2 d−1, respectively. Generally low net in situ DOC and DON fluxes as well as steepening of spectral slope (S) of CDOM and accumulation of humic-like FDOM at the near-bottom waters over time indicated active microbial DOM utilization at the sediment–water interface, potentially stimulated by nitrate (NO3−) and nitrite (NO2−). The microbial DOC utilization rates, estimated in our study, may be sufficient to support denitrification rates of 0.2–1.4 mmol m−2 d−1, suggesting that sediment release of DOM contributes substantially to nitrogen loss processes in the ETSP off Peru.