Organic carbon accumulation in British saltmarshes

Saltmarshes are a crucial component of the coastal carbon (C) system and provide a natural climate regulation service through the accumulation and long-term storage of organic carbon (OC) in their soils. These coastal ecosystems are under growing pressure from a changing climate and increasing anthropogenic disturbance. To manage and protect these ecosystems for C and to allow their inclusion in emissions and natural-capital accounting, as well as carbon markets, accurate and reliable estimates of OC accumulation are required. However, globally, such data are rare or of varying quality. Here, we quantify sedimentation rates and OC densities for 21 saltmarshes in Great Britain (GB). We estimate that, on average, saltmarshes accumulate OC at a rate of 110.88 ± 43.12 g C m-2 yr-1. This is considerably less than widely applied global saltmarsh averages. It is therefore highly likely that the contribution of northern European saltmarshes to global saltmarsh OC accumulation has been significantly overestimated. Taking account of the climatic, geomorphological, oceanographic, and ecological characteristics of all GB saltmarshes and the areal extent of different saltmarsh zones, we estimate that the 451.65 km2 of GB saltmarsh accumulates 46,563 ± 4353 t of OC annually. These low OC accumulation rates underline the importance of the 5.20 ± 0.65 million tonnes of OC already stored in these vulnerable coastal ecosystems. Going forward the protection and preservation of the existing stores of OC in GB saltmarshes must be a priority for the UK as this will provide climate benefits through avoided emissions several times more significant than the annual accumulation of OC in these ecosystems.

Global dataset of soil organic carbon in tidal marshes

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha-1 in the top 30 cm and 231 ± 134 Mg SOC ha-1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Consistently dated Atlantic sediment cores over the last 40 thousand years

Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Impacts of ocean acidification on intertidal benthic foraminiferal growth and calcification

Foraminifera are expected to be particularly susceptible to future changes in ocean carbonate chemistry as a function of increased atmospheric CO₂. Studies in an experimental recirculating seawater system were performed with a dominant benthic foraminiferal species collected from intertidal mudflats. We investigated the experimental impacts of ocean acidification on survival, growth/calcification, morphology and the biometric features of a calcareous species Elphidium williamsoni. Foraminifera were exposed for 6 weeks to four different pH treatments that replicated future scenarios of a high CO₂ atmosphere resulting in lower seawater pH. Results revealed that declining seawater pH caused a decline in foraminiferal survival rate and growth/calcification (mainly through test weight reduction). Scanning electron microscopy image analysis of live specimens at the end of the experimental period show changes in foraminiferal morphology with clear signs of corrosion and cracking on the test surface, septal bridges, sutures and feeding structures of specimens exposed to the lowest pH conditions. These findings suggest that the morphological changes observed in shell feeding structures may serve to alter: (1) foraminiferal feeding efficiency and their long-term ecological competitiveness, (2) the energy transferred within the benthic food web with a subsequent shift in benthic community structures and (3) carbon cycling and total CaCO₃ production, both highly significant processes in coastal waters. These experimental results open-up the possibility of modelling future impacts of ocean acidification on both calcification and dissolution in benthic foraminifera within mid-latitude intertidal environments, with potential implications for understanding the changing marine carbon cycle.

Fjord systems and archives: a review

Fjords are glacially over-deepened semi-enclosed marine basins, typically with entrance sills separating their deep waters from the adjacent coastal waters which restrict water circulation and thus oxygen renewal. The location of fjords is principally controlled by the occurrence of ice sheets, either modern or ancestral. Fjords are therefore geomorphological features that represent the transition from the terrestrial to the marine environment and, as such, have the potential to preserve evidence of environmental change. Typically, most fjords have been glaciated a number of times and some high-latitude fjords still possess a resident glacier. In most cases, glacial erosion through successive glacial/interglacial cycles has ensured the removal of sediment sequences within the fjord. Hence the stratigraphic record in fjords largely preserves a glacial-deglacial cycle of deposition over the last 18 ka or so. Sheltered water and high sedimentation rates have the potential to make fjords ideal depositional environments for preserving continuous records of climate and environmental change with high temporal resolution. In addition to acting as high-resolution environmental archives, fjords can also be thought of as mini-ocean sedimentary basin laboratories. Fjords remain an understudied and often neglected sedimentary realm. With predictions of warming climates, changing ocean circulation and rising sea levels, this volume is a timely look at these environmentally sensitive coastlines.

Supplementary material:

The Glossary is available at: http://www.geolsoc.org.uk/SUP18440.

Using seismic facies and pollen analyses to evaluate climatically driven change in a Scottish sea loch (fjord) over the last 20 ka

Loch Sunart is a glacially over-deepened sea loch (fjord) on the west coast of Scotland, UK. The loch is divided into three sub-basins, separated by relatively shallow and narrow sills. A programme of data collection including high-resolution bathymetric sonar and sub-bottom seismic surveys were conducted in the loch as part of an investigation into the sedimentological and climatic change signatures preserved in western sea lochs since the Last Glacial Maximum. Very-high-resolution sub-bottom profiles were obtained using the SEISTEC boomer system. The seismic profiles revealed an igneous and metamorphic basement covered by a 10–70 m thick sediment sequence. Five different acoustic facies were recognized and interpreted in terms of glacial activity, ice retreat and subsequent Holocene sedimentation. These facies have been correlated to sediments sampled in a radiocarbon-dated 12 m long giant piston core (MD04-2833) acquired from the main basin of Loch Sunart. Pollen analyses conducted along the length of the core, together with 14C dating, indicate a complex series of palaeoclimate changes in the loch. In particular, five distinct cooling events have been recognized c. 9.8, 8.2, 5.8, 1.2 cal ka BP and 771–1211 cal a BP (possibly the Little Ice Age), corresponding to phases of Holocene rapid climate change.

Millennial and sub-millennial-scale variability in sediment colour from the Barra Fan, NW Scotland: implications for British ice sheet dynamics

Sediment colour, together with other proxy data, provides a novel, rapid and non-destructive tool in the investigation of glacier-influenced sedimentation on the Barra Fan, NW Scotland. Lightness (L*) and reflectance (400–700 nm) measurements at this site provide a quantitative estimate of changes in calcium carbonate and clay content. Interstadials are carbonate-rich/clay-poor (higher L* and reflectivity), whereas stadials are carbonate-poor/clay-rich (lower L* and reflectivity). Detailed sedimentological investigations suggest that the last British Ice Sheet (BIS) extended to the outer continental shelfbreak shortly after 30 ka bp. This climatic response of the BIS to global cooling at the Marine Isotope Stage (MIS) 3 – 2 transition marks a significant increase in sediment delivery to the Barra Fan. Prior to 30 ka bp, strong Dansgaard/Oeschger (D/O) cyclicity dominates the record. After 30 ka bp, shorter periodicities prevailed as the BIS reached its maximum extent. Glacier dynamics plays a significant role in the delivery of ice-rafted debris (IRD) across this margin, highlighting the inherent difficulties of correlating millennial-scale IRD events when the IRD is derived from different ice sheets. An event stratigraphy based upon carbonate-rich interstadials provides a more robust means of amphi-Atlantic correlation during this interval.