Lack of synsedimentary chemical alteration in polar carbonates (Ross Sea, Antarctica): Resolution of a conundrum

Abstract

ABSTRACTAlthough rare in space and time, skeletal carbonates deposited on polar shelves hold great potential for improving understanding of the oceanography of the high latitudes. Low temperatures, low carbonate saturation states, and strong seasonality govern not only the nature of carbonate communities, but also how their deposits translate into the rock record. To understand the effects of early seafloor processes on preservation, we investigated late Quaternary carbonates recovered in piston cores from the Ross Sea, Antarctica. Sediments are unconsolidated skeletal gravels and sands that mantle areas of the outer shelf swept by strong bottom currents. Deposits are dominated locally by either stylasterine hydrocorals, barnacles, or bryozoans, which comprise assemblages with strong similarities to modern benthic communities. Radiocarbon ages indicate that carbonate factories were most prolific during the lead-up to the Last Glacial Maximum (Tartanian), when sediment input was minimized, and have been mostly dormant since. Results show that synsedimentary alteration is not substantially different in the temperate and polar realms with the significant exception of chemical diagenesis. As is common in temperate deposits, skeletal grains undergo disarticulation, fracturing, abrasion, and intense bioerosion. By contrast, cementation is absent and rare aragonite grains are preserved, indicating that taphonomic loss is not as prevalent as in temperate deposits. Primary skeletal microstructures and stable-isotope compositions are preserved, indicating that chemical alteration of grains is negligible. The preservation of aragonite in polar settings is herein attributed to low rates of organic-matter burial and very low temperatures, which strongly limit microbial activity. These factors allow interstitial waters to remain weakly supersaturated with respect to aragonite. Comparison with Permian analogs indicates that lithification is delayed until deposits reach burial depths at which chemical compaction proceeds. The ultimate end product is limestone with prominent compaction features and a tightly packed fabric. Calcitic skeletal material can retain primary geochemical compositions through the lithification process, although growth of burial cement in intraparticle porosity complicates selective sampling of unaltered material. In providing a cold-water end member for the spectrum of synsedimentary diagenetic processes, results highlight specific differences that should be accounted for when interpreting the deposits of polar, cold-water carbonate systems.