Carbonate mud is a major constituent of recent marine carbonate sediments and of ancient limestones, which contain unique records of changes in ocean chemistry and climate shifts in the geological past. However, the origin of carbonate mud is controversial and often problematic to resolve. Here we show that tropical marine fish produce and excrete various forms of precipitated (nonskeletal) calcium carbonate from their guts ("low" and "high" Mg-calcite and aragonite), but that very fine-grained (mostly <2 μm) high Mgcalcite crystallites (i.e., >4 mole % MgCO 3 ) are their dominant excretory product. Crystallites from fish are morphologically diverse and species-specific, but all are unique relative to previously known biogenic and abiotic sources of carbonate within open marine systems. Using site specific fish biomass and carbonate excretion rate data we estimate that fish produce ∼6.1 × 10 6 kg CaCO 3 ∕year across the Bahamian archipelago, all as mud-grade (the <63 μm fraction) carbonate and thus as a potential sediment constituent. Estimated contributions from fish to total carbonate mud production average ∼14% overall, and exceed 70% in specific habitats. Critically, we also document the widespread presence of these distinctive fish-derived carbonates in the finest sediment fractions from all habitat types in the Bahamas, demonstrating that these carbonates have direct relevance to contemporary carbonate sediment budgets. Fish thus represent a hitherto unrecognized but significant source of fine-grained carbonate sediment, the discovery of which has direct application to the conceptual ideas of how marine carbonate factories function both today and in the past. marine teleost | fish intestine | carbonate production M arine carbonates contain unique records of changes in ocean chemistry, biogeochemical cycling, and benthic and pelagic ecology (1), and therefore provide vital information on climate shifts in the geological past. A distinctive and often volumetrically important component of these sediments is carbonate mud (the <63 μm sediment fraction). However, the origins of both aragonitic and Mg-calcite carbonate muds remains a topic of long-standing debate (2, 3). Indeed, where attempts have been made to quantify sources of the fine sediment fraction a high proportion remains of unknown origin (e.g., up to 40% in Bahamian sediments and between 28 and 36% in Belize lagoon sediments) (4, 5). This problem arises in part because, with the exception of inorganic carbonate precipitation (e.g., the carbonate "whiting" controversy) (3, 6-8), the processes of carbonate mud production necessarily invoke the degradation of larger bioclasts (skeletal fragments of marine organisms) to produce mud-grade carbonate, and/or grain recrystallization to produce high Mg-calcite muds (9-11). Thus attempts to determine primary mud sources and production budgets are often hampered because of grain obliteration. The mineralogical composition of the mud fraction of modern tropical carbonate sediment is also very variable between s...

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Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob's Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob's Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive 13 C excursion in the terminal Ordovician. The architecture of the 13 C curve from Dob's Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dob's Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dob's Linn as a boundary stratotype.

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This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (&lt;1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were &gt;230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite-cemented breccias at much shallower depths than previously realized.