191-4 Anomalous Cold-Water Carbonate Hardgrounds from the Alpha Ridge, Arctic Ocean: Formation, Composition, and Implications

Session: Aspects of Carbonates (Posters)

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Carbonate hardgrounds represent periods of intense synsedimentary cementation of the seafloor. They are typically composed of high Mg and aragonitic cements and tend to form in tropical latitudes characterized by warm, shallow marine environments saturated with calcium carbonate. However, carbonate hardgrounds recovered from the Arctic Ocean’s Alpha Ridge (depths 1555–2575 m) during a 2016 polar expedition co-organized by the Geological Survey of Canada and the Swedish Polar Research Secretariat, challenge this paradigm. Palynological analysis, primarily based on foraminifera, constrains the samples to a Quaternary age (<0.4 Ma). We employed lithological, petrographic, x-ray diffraction, and geochemical (stable isotope) methodologies to try and determine how these cold-water, high-latitude, deep water carbonate crusts formed in such an assumedly unfavourable environment. Hand sample and petrographic analyses along with x-ray diffraction, demonstrate that these Arctic hardgrounds are composed of detrital clay, fine-grained quartz, and pebble-sized dropstones cemented together by authigenic low Mg calcite. Ichnofossils in the form of burrows are present and include Thalassinoides and Planolites burrows, along with borings of clionid sponges. The hardground surfaces are encrusted with serpulid worm tubes and stained with manganese. Stable carbon isotope values for the carbonate hardgrounds (2.3‰ to 2.9‰) are typical of Arctic seawater and are not in the range of carbonate crusts associated with methane. Stable oxygen isotope values range from 3.8‰ to 4.0‰, suggesting a precipitation temperature between -2.2°C and -3.7°C, which is consistent with bottom water temperatures recorded during the 2016 expedition. We propose that these polar, cold-temperature, low Mg carbonate hardgrounds formed from waters saturated with calcium carbonate that were transported into the region. These foreign waters likely became saturated by coming in contact with Paleozoic basement limestones, after which they travelled through fault pathways associated with the Alpha-Mendeleev Fault Zone. Once expelled into the water column on the Alpha Ridge, the waters cooled and interacted with sediment on the ridge leading to the precipitation of micritic cements and the formation of the carbonate hardgrounds. The sequence of biogenic activity indicates burrowing predated lithification, followed by sponge boring and serpulid encrustation post-cementation. This study highlights an unusual cold-water formation pathway for carbonate hardgrounds in high-latitude, deep-marine settings.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7552

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