Cold-water carbonates in The Great Land: intertidal carbonate factories of Kachemak Bay, Alaska

Low tides revealed striking white beaches among dark volcaniclastic rocks on islands within Kachemak Bay, Alaska, USA. Upon further investigation, we found that these beaches are rich with sands and gravels sourced from carbonate-producing invertebrates. Kachemak Bay is a subarctic fjord in southcentral Alaska with cold seawater temperatures (~5–11°C), glaciers, rivers, and extreme tides (average daily range ~6 m, annual maximum range ~9 m). This region is tectonically active and is being uplifted by isostatic rebound and the descending Pacific Plate of the Aleutian subduction zone making it an unexpected location for carbonate sedimentation. In this study, we characterized grain sizes and compositions of these mixed lithic-carbonate sediments; results may be used as an analog to other cool-water deposits. We investigated five islands and collected sediment samples from beach surfaces and from layers within shallowly dug trenches (<1m). These sediments contain medium sands to coarse gravels. Observed strata in trenches indicated grain sorting and planar bedding characteristics of foreshore deposits worldwide. We used a combination of stereoscopes and thin section petrography to study grain compositions. The sediments contain lithic fragments and heterozoan skeletal carbonates dominated by barnacle plates and mussel shells, with minor contributions from infaunal bivalves, echinoderms, bryozoans, gastropods, and chitons. These invertebrates are abundant in adjacent rocky intertidal habitats, which likely serve as local carbonate factories. Both carbonate and lithic grains show signs of physical abrasion, presumably caused by waves, storms, and tidal currents. Many of the carbonate grains are also riddled with microborings, recording the physical and chemical weathering activities of endolithic algae and sponges. Kachemak Bay’s active heterozoan carbonate sedimentation offers a modern analog for mixed lithic-carbonate foreshore deposits and illustrates the role of local carbonate production and physical reworking in cool-water systems.