ARAGONITE PRECIPITATION IN THE LATE ORDOVICIAN ‘CALCITE SEA’ — EVIDENCE FROM A MULTI-PROXY GEOCHEMICAL APPROACH

A positive excursion in δ²⁶Mg values (2–3‰) is observed, recorded in a carbonate succession in the Monitor Range, Nevada, spanning the Late Ordovician (Hirnantian) glaciation event. The increase in δ²⁶Mg values is synchronous with previously detailed δ^44/40Ca and δ¹³C excursions in the same section, an observation at odds with the large difference in residence times between Mg, Ca, and C, suggesting that the isotopic shifts are not the result of global-ocean cycling. A mixing analysis reveals that the positive shift in δ²⁶Mg values in the whole-rock carbonate is due to a small increase in dolomite abundance. The analysis is also used to determine original limestone δ²⁶Mg values in marine carbonates, revealing stratigraphic changes in the δ²⁶Mg values of the carbonate deposited during the glacial sea-level lowstand, and the pre-glacial and post-glacial sea-level highstands. The limestone end-member exhibits a higher δ²⁶Mg value for carbonate deposited in the sea-level lowstand, consistent with precipitation of primary aragonite during the Hirnantian glaciation in this tropical-shelf setting, and lower δ²⁶Mg values before and after the glaciation, which is consistent with the precipitation of calcite during the sea-level highstands. However, the effects of diagenesis on δ²⁶Mg values in carbonate sediment are difficult to predict, therefore Mg isotopes alone do not permit a conclusive determination of primary carbonate polymorph mineralogy. A synchronous negative shift in δ^44/40Ca values, positive shift in δ¹³C values, and intermittently high Sr/Ca ratios recorded in the carbonates deposited during the sea-level lowstand interval add support to the interpretation of aragonite deposition during the glaciation. Previous laboratory experiments have also demonstrated that aragonite can precipitate in seawater with the chemistry of a ‘calcite sea’ at temperatures above 20–23°C, which was likely common at low latitudes in the early Paleozoic. This study demonstrates the utility of applying a multi-proxy geochemical approach using Mg, Ca, and C isotopes, and elemental Sr/Ca ratios to reveal aragonite that has inverted to calcite over geologic time. It also cautions against misinterpreting facies-dependent changes in carbonate mineralogy as genuine records of secular variation in seawater chemistry.