Platelet ice, the Southern Ocean's hidden ice: a review

Mario Hoppmann,David N Thomas,Stefan Jendersie,Gerhard S Dieckmann,Maren E Richter,Inga J Smith,Patricia J Langhorne

doi:10.1017/aog.2020.54

Abstract

AbstractBasal melt of ice shelves is not only an important part of Antarctica's ice sheet mass budget, but it is also the origin of platelet ice, one of the most distinctive types of sea ice. In many coastal Antarctic regions, ice crystals form and grow in supercooled plumes of Ice Shelf Water. They usually rise towards the surface, becoming trapped under an ice shelf as marine ice or forming a semi-consolidated layer, known as the sub-ice platelet layer, below an overlying sea ice cover. In the latter, sea ice growth consolidates loose crystals to form incorporated platelet ice. These phenomena have numerous and profound impacts on the physical properties, biological processes and biogeochemical cycles associated with Antarctic fast ice: platelet ice contributes to sea ice mass balance and may indicate the extent of ice-shelf basal melting. It can also host a highly productive and uniquely adapted ecosystem. This paper clarifies the terminology and reviews platelet ice formation, observational methods as well as the geographical and seasonal occurrence of this ice type. The physical properties and ecological implications are presented in a way understandable for physicists and biologists alike, thereby providing the background for much needed interdisciplinary research on this topic.

Highlights

This paper tells the story of a most interesting form of sea ice, which many people may never have heard of
Its presence results from an interaction between the ocean, ice shelves and immobile, landfast sea ice attached to the latter (Fraser and others, 2012)
Even in close proximity to ice-shelf cavities cold enough to produce Ice Shelf Water (ISW), platelet ice may be absent for all or part of the year. This can be due to (a) ISW emerging at depth; (b) ISW mixing with warmer surface water masses before exiting the cavity (Jacobs and others, 1992; Hattermann and others, 2012); (c) in situ supercooling in the ISW plume being relieved through latent heat release from marine or anchor ice formation at depth (Sections 3.5 and 3.8) or (d) the presence of a polynya with intense brine rejection, as hypothesised by Langhorne and others (2015)

Summary

Introduction

This paper tells the story of a most interesting form of sea ice, which many people may never have heard of. Its presence results from an interaction between the ocean, ice shelves (floating extensions of the continental ice sheet) and immobile, landfast sea ice attached to the latter (Fraser and others, 2012). In contrast to regular sea ice, which initially forms and grows at the ocean surface by heat conduction to the atmosphere (Stefan, 1891), platelet ice originates from supercooling of sea water deeper in the water column, inside the ice-shelf cavity. There, basal ablation of the ice shelf cools and freshens the surrounding ocean water, which becomes more buoyant, and ascends towards the surface. The accompanying pressure relief can cause this water to have a potential temperature below the in situ freezing point, a condition which is called in situ supercooling (Foldvik and Kvinge, 1974)

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