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The response of tidewater glaciers to ocean warming remains a key uncertainty in sea level rise predictions. Here we use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rate. While melting toward the center of the terminus causes only a localized increase in mass loss, melting near the lateral margins triggers increased calving across the width of the glacier, causing the terminus to retreat at several times the width‐averaged melt rate. This occurs because melting near the margins has a greater disruptive impact on the compressive stress arch that transfers resistance from the side walls to the body of the glacier. We suggest that the rate of terminus advance or retreat may thus be governed by the difference between ice velocity and submarine melting in the slow‐flowing zones away from the glacier center. The rapid retreat of tidewater glaciers (i.e., glaciers that drain directly into the ocean) has become an increasing source of concern in recent years. Increased melting of the submerged parts of these glaciers by warming ocean waters is thought to be an important driver of this retreat, but exactly how glaciers respond to this submarine melting remains unclear. Using a numerical model, we find that tidewater glaciers may be most sensitive to melting close to the valleysides, which reduces structural support for the central section of the glacier and so triggers an increase in iceberg calving. By better constraining the relationships between submarine melting, calving, and glacier retreat, our findings allow improved prediction of the ice loss expected from tidewater glaciers as the climate continues to warm. We use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rateThe glacier is most sensitive to melting near the lateral margins, which triggers increased calving across the width of the terminusTerminus retreat may thus be paced by the difference between ice velocity and submarine melting across slow flowing marginal zones

Iceberg calving accounts for around half of all mass loss from both the Greenland and Antarctic ice sheets. The diverse nature of calving and its complex links to both internal dynamics and climate make it challenging to incorporate into models of glaciers and ice sheets. Here we present results from a new open‐source 3‐D full‐Stokes calving model developed in Elmer/Ice. The calving model implements the crevasse depth criterion, which states that calving occurs when surface and basal crevasses penetrate the full thickness of the glacier. The model also implements a new 3‐D rediscretization approach and a time‐evolution scheme which allow the calving front to evolve realistically through time. We test the model in an application to Store Glacier, one of the largest outlet glaciers in West Greenland, and find that it realistically simulates the seasonal advance and retreat when two principal environmental forcings are applied. These forcings are (1) submarine melting in distributed and concentrated forms and (2) ice mélange buttressing. We find that ice mélange buttressing is primarily responsible for Store Glacier's seasonal advance and retreat. Distributed submarine melting prevents the glacier from forming a permanent floating tongue, while concentrated plume melting has a disproportionately large and potentially destabilizing effect on the calving front position. Our results also highlight the importance of basal topography, which exerts a strong control on calving, explaining why Store Glacier has remained stable during a period when neighboring glaciers have undergone prolonged interannual retreat. Most freshwater on our planet is stored as ice in the ice sheets of Greenland and Antarctica. The ice sheet in Greenland is currently losing mass at a rate that is equal to 1 mm/year of sea level rise. Around half of the ice loss in Greenland is lost through icebergs released into the sea through a process called “calving.” Iceberg calving is poorly understood and has so far not been included in 3‐D models needed to predict sea level rise. Recent studies show that warming of the air and ocean is linked to more calving, but we still do not understand these links sufficiently to make predictions. This study presents a new computer model, which is the first to simulate glacier flow and iceberg calving in 3‐D, and investigates calving at Store Glacier in Greenland. We test the model by reproducing the present‐day seasonal cycle of the glacier and find that fractures forming on the surface as well as the bottom of the glacier control the calving rate. We show that the calving rate is influenced by submarine melting of the calving ice front as well as by ice mélange, which is a mixture of icebergs and sea ice forming in front of the glacier in winter. A 3‐D full‐Stokes calving model was implemented in Elmer/Ice and applied to Store Glacier in West GreenlandThe model reproduces the seasonal characteristics of ice flow and calving without tuningIce mélange and submarine melting link calving to the ocean, strongly modulated by basal topography

Tephrochronology allows the establishment of ‘isochrons’ between marine, lacustrine, terrestrial and ice cores, typically based on the geochemical fingerprint of the tephra. The development of cryptotephrochronology has revealed a vast inventory of isochrons which hold the potential to improve stratigraphic correlation and identify systemic leads and lags in periods of rapid climate change. Unfortunately, bioturbation acts to blur these isochrons, reducing the temporal resolution in marine and lacustrine records. In order to better resolve these event horizons, we require a better understanding of bioturbative processes, and the depth and time over which they operate. To this end, an ash fall event was simulated on the intertidal zone of the Eden Estuary, Fife, Scotland and sediment cores were collected over 10 days. A novel approach to tephra quantification was developed, using the imaging software ImageJ. Our results showed limited bioturbation (mixed depth=18 mm), most likely owing to the fine grain size, low-energy environment and the resulting faunal composition of the sediments. These results imply a strong ecological control on bioturbation, and suggest that inferences may be made about palaeoenvironments from the observed bioturbation profiles.supplementary material:The ImageJ macro used in this study, as well as raw tephra concentration data and details of the method validation are available at http://www.geolsoc.org.uk/SUP18725.