Modeling Antarctic tides in response to ice shelf thinning and retreat

Tides play an important role in ice sheet dynamics by modulating ice stream velocity, fracturing, and moving ice shelves and mixing water beneath them. Any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability. Here, we show that removal or reduction in extent and/or thickness of the Ross and Ronne-Filchner ice shelves would have a significant impact on the tides around Antarctica. The Ronne-Filchner appears particularly vulnerable, with an increase in M2 amplitude of over 0.5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf/sheet dynamics. These results highlight the importance of understanding tidal feedbacks on ice shelves/streams due to their influence on ice sheet dynamics

Global tidal impacts of large-scale ice-sheet collapses

Tide model output for "Wilmes et al., (2017), Global tidal impacts of large-scale ice-sheet collapses, JGR Oceans" together with the Matlab files needed to read the model binary files Please refer to the publications for details on the run setup. h0.* contains elevation output; M2 elevations can be read in Matlab using [h,th_lim,ph_lim] = h_in(filename,1); where h is tidal elevation (abs(h) gives amplitudes and angle(h) gives phase), th_lim gives latitude limits in degs N and ph_lim longitude limits in degs E u0.* contains tidal transport output; M2 transports can be read in Matlab using [u,v,th_lim,ph_lim] = u_in(filename,1); where u and v are transports in x and y direction (real(u)/hz gives tidal current strength) grid* contains the bathymetry; can be read in Matlab using [ll_lims,hz,mz,iob] = grd_in(filename); where ll_lims gives lon and lat limits, hz is water depth, mz is the land-sea mask (0 is land, 1 is water), and iob are open boundary nodes *.it_m2_k1_00.0kyrBP_ish_no0.1sal_191322_sal4 - CTRL; bathymetry: grid_etssib_1_8_paleo_glob_ice_shelves *.it_m2_k1_00.0kyrBP_ish_5mSLR_vw_no0.1sal_191333_sal4 - 5m SLR; bathymetry: grid_etssib_1_8_paleo_glob_ice_shelves_5mSLR_vw *.it_m2_k1_00.0kyrBP_ish_7mSLR_vw_no0.1sal_191336_sal4 - 7m SLR; bathymetry: grid_etssib_1_8_paleo_glob_ice_shelves_7mSLR_vw *.it_m2_k1_1_8th_00.0kyrBP_12mSLR_vw_7048752_sal4 - 12m SLR; bathymetry: grid_etssib_1_8_paleo_glob_ice_shelves_12mSLR_vw *.it_m2_k1_00.0kyrBP_no_wais_fp_5mSLR_vw_no0.1sal_191326_sal4 - No WAIS; bathymetry: grid_etssib_1_8_glob_no_wais_SLR_fingerprint_5m_EEV_vw *.it_m2_k1_00.0kyrBP_no_gris_fp_7mSLR_vw_no0.1sal_191331_sal4 - No GIS; bathymetry: grid_etssib_1_8_glob_no_gris_SLR_fingerprint_7m_EEV_vw *.it_m2_00.0kyrBP_no_wais_gis_fp_vw_375526_sal4 - No WAIS & No GIS; bathymetry: grid_etssib_1_8_glob_no_wais_gris_SLR_fingerprint_12m_EEV_v

Changes in wave climate over the northwest European shelf seas during the last 12,000 years

Because of the depth attenuation of wave orbital velocity, wave-induced bed shear stress is much more sensitive to changes in total water depth than tidal-induced bed shear stress. The ratio between wave- and tidal-induced bed shear stress in many shelf sea regions has varied considerably over the recent geological past because of combined eustatic changes in sea level and isostatic adjustment. In order to capture the high-frequency nature of wind events, a two-dimensional spectral wave model is here applied at high temporal resolution to time slices from 12 ka BP to present using paleobathymetries of the NW European shelf seas. By contrasting paleowave climates and bed shear stress distributions with present-day conditions, the model results demonstrate that, in regions of the shelf seas that remained wet continuously over the last 12,000 years, annual root-mean-square (rms) and peak wave heights increased from 12 ka BP to present. This increase in wave height was accompanied by a large reduction in the annual rms wave- induced bed shear stress, primarily caused by a reduction in the magnitude of wave orbital velocity penetrating to the bed for increasing relative sea level. In regions of the shelf seas which remained wet over the last 12,000 years, the annual mean ratio of wave- to (M-2) tidal-induced bed shear stress decreased from 1 (at 12 ka BP) to its present-day value of 0.5. Therefore compared to present- day conditions, waves had a more important contribution to large-scale sediment transport processes in the Celtic Sea and the northwestern North Sea at 12 ka BP

Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream

We report 23 cosmogenic isotope exposure ages (10Be and 36Cl) relating to the maximum extent and deglaciation chronology of the Irish Sea Ice Stream (ISIS), which drained the SW sector of the last British-Irish Ice Sheet. These show that the ISIS failed to reach the Preseli Hills of North Pembrokeshire yet extended southwards to impinge on northern Isles of Scilly (50°N) during the last glacial maximum. Four samples from western Anglesey demonstrate deglaciation of the southern Irish Sea Basin by c. 20-18 ka, and two from the Llŷn Peninsula in northwest Wales, if valid, suggest deglaciation by c. 23-22 ka followed by gradual oscillatory northwards retreat of the ice margin for over 3000 years. An alternative interpretation of our data suggests that ice reached Scilly as late as 22-21 ka then retreated 450 km northwards within the following three millennia, possibly in response to sea level rise and/or intrinsic reorganisation within the last British-Irish Ice Sheet. Samples from upland source areas of the ISIS in NW England and SW Scotland produced exposure ages ≤14.3 ka, suggesting possible persistence of ice in such areas into the Lateglacial Interstade of 14.7-12.9 ka

South Georgia marine productivity over the past 15 ka and implications for glacial evolution

The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia’s response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3cal.kyrBP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2cal.kyrBP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5cal.kyrBP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1cal.kyrBP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1cal.kyrBP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene

Biological and climate controls on North Atlantic marine carbon dynamics over the last millennium: Insights from an absolutely-dated shell based record from the North Icelandic Shelf

Given the rapid increase in atmospheric carbon dioxide concentrations (pCO2) over the industrial era, there is a pressing need to construct long‐term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the past biological and climate controls on the carbon isotopic (δ13Cshell) composition of the North Icelandic shelf waters over the last millennium, derived from the shells of the long‐lived marine bivalve mollusk Arctica islandica. Variability in the annually resolved δ13Cshell record is dominated by multidecadal variability with a negative trend (−0.003 ± 0.002‰ yr−1) over the industrial era (1800–2000 Common Era). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (δ13C of CO2) derived from the burning of fossil fuels. Comparison of the δ13Cshell record with Contemporaneous proxy archives, over the last millennium, and instrumental data over the twentieth century, highlights that both biological (primary production) and physical environmental factors, such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and sea surface temperature and salinity of the subpolar gyre, are the likely mechanisms that contribute to natural variations in seawater δ13C variability on the North Icelandic shelf. Contrasting δ13C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated marine Suess effect signal at this locality

Natural drivers of multidecadal Arctic sea ice variability over the last millennium

This is the final version. Available from Nature Research via the DOI in this record.The climate varies due to human activity, natural climate cycles, and natural events external to the climate system. Understanding the different roles played by these drivers of variability is fundamental to predicting near-term climate change and changing extremes, and to attributing observed change to anthropogenic or natural factors. Natural drivers such as large explosive volcanic eruptions or multidecadal cycles in ocean circulation occur infrequently and are therefore poorly represented within the observational record. Here we turn to the first high-latitude annually-resolved and absolutely dated marine record spanning the last millennium, and the Paleoclimate Modelling Intercomparison Project (PMIP) Phase 3 Last Millennium climate model ensemble spanning the same time period, to examine the influence of natural climate drivers on Arctic sea ice. We show that bivalve oxygen isotope data are recording multidecadal Arctic sea ice variability and through the climate model ensemble demonstrate that external natural drivers explain up to third of this variability. Natural external forcing causes changes in sea-ice mediated export of freshwater into areas of active deep convection, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) and thereby northward heat transport to the Arctic. This in turn leads to sustained anomalies in sea ice extent. The models capture these positive feedbacks, giving us improved confidence in their ability to simulate future sea ice in in a rapidly evolving Arctic.Natural Environment Research Council (NERC)Natural Environment Research Council (NERC)Natural Environment Research Council (NERC)Leverhulme TrustAustralian Research CouncilEuropean Union’s Horizon 202

The Marine Radiocarbon Bomb Pulse across the Temperate North Atlantic: A Compilation of Δ14C Time Histories from Arctica islandica Growth Increments

Marine radiocarbon bomb-pulse time histories of annually resolved archives from temperate regions have been underexploited. We present here series of Δ14C excess from known-age annual increments of the long-lived bivalve mollusk Arctica islandica from 4 sites across the coastal North Atlantic (German Bight, North Sea; Troms⊘, north Norway; Siglufjordur, north Icelandic shelf; Grimsey, north Icelandic shelf) combined with published series from Georges Bank and Sable Bank (NW Atlantic) and the Oyster Ground (North Sea). The atmospheric bomb pulse is shown to be a step-function whose response in the marine environment is immediate but of smaller amplitude and which has a longer decay time as a result of the much larger marine carbon reservoir. Attenuation is determined by the regional hydrographic setting of the sites, vertical mixing, processes controlling the isotopic exchange of 14C at the air-sea boundary, 14C content of the freshwater flux, primary productivity, and the residence time of organic matter in the sediment mixed layer. The inventories form a sequence from high magnitude-early peak (German Bight) to low magnitude-late peak (Grimsey). All series show a rapid response to the increase in atmospheric Δ14C excess but a slow response to the subsequent decline resulting from the succession of rapid isotopic air-sea exchange followed by the more gradual isotopic equilibration in the mixed layer due to the variable marine carbon reservoir and incorporation of organic carbon from the sediment mixed layer. The data constitute calibration scries for the use of the bomb pulse as a high-resolution dating tool in the marine environment and as a tracer of coastal ocean water masses