Articles | Volume 10, issue 5
Research article
13 Sep 2016
Research article |  | 13 Sep 2016

Grounding and calving cycle of Mertz Ice Tongue revealed by shallow Mertz Bank

Xianwei Wang, David M. Holland, Xiao Cheng, and Peng Gong

Abstract. A recent study, using remote sensing, provided evidence that a seafloor shoal influenced the 2010 calving event of the Mertz Ice Tongue (MIT), by partially grounding the MIT several years earlier. In this paper, we start by proposing a method to calculate firn air content (FAC) around Mertz from seafloor-touching icebergs. Our calculations indicate the FAC around Mertz region as 4.87 ± 1.31 m. We then design an indirect method of using freeboard and sea surface height data extracted from ICESat/GLAS, FAC, and relatively accurate seafloor topography to detect grounding sections of the MIT between 2002 and 2008 and analyze the process of grounding prior to the calving event. By synthesizing remote sensing data, we point out that the grounding position was localized northeast of the Mertz ice front close to the Mertz Bank. The grounding outlines of the tongue caused by the Mertz Bank are extracted as well. From 2002 to 2008, the grounding area increased and the grounding became more pronounced. Additionally, the ice tongue could not effectively climb over the Mertz Bank in following the upstream ice flow direction and that is why MIT rotated clockwise after late 2002. Furthermore, we demonstrate that the area-increasing trend of the MIT changed little after calving (∼  36 km2 a−1), thus allowing us to use remote sensing to estimate the elapsed time until the MIT can reground on and be bent by the shoal. This period is approximately 70 years. Our observations suggest that the calving of the MIT is a cyclical process controlled by the presence of the shallow Mertz Bank location and the flow rate of the tongue. This calving cycle also explains the cyclic variations in sea-surface conditions around the Mertz detected by earlier studies.

Short summary
MIT was reported to have calved subsequent to being rammed by a large iceberg. However from remote sensing, the ice fronts being rammed did not move out first which led us to detect the influence of seafloor on instability of MIT. Using Firn Air Content extracted from slightly grounded icebergs, laser altimetry, remote sensing, and seafloor topography data, grounding of the MIT caused by Mertz Bank is extracted. Mertz Bank is confirmed to control calving of the MIT at a cycle of ~70 years.