Diagenesis and Compositional Partitioning of Quaternary Cool-water Carbonate Aeolianites: Southeastern Australia

Middle to late Pleistocene stranded carbonate sea beaches, adjacent aeolian dunes, and Pleistocene aeolianites together with Holocene marginal marine calcareous sediments across southeastern Australia contain the longest record of such deposition in the modern world, stretching back to at least similar to 900 ky (MIS 23). The grains are/were derived from a suite of cool-water, heterozoan carbonate sediments produced on the adjacent offshore shelf. Most modern shelf carbonate sediments in the region today are aragonite-rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record. Beach-dune sediments are derived mainly from the shallow < 30 meters deep adjacent seafloor. They are mollusk-rich but with most bryozoan particles having been destroyed in the surf zone. The mollusks are dominantly aragonite. These nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite-poor open-shelf sediments. Aragonite skeletal fragments in the dunes progressively dissolve with time under a semi-arid climate and produce diagenetic low-magnesium calcite meteoric cements, a well-known process common in tropical, aragonitic Pleistocene aeolianites. The loss of metastable minerals with time is matched by a progressive and well-understood change in stable isotopes. Such alteration in this environment, however, seems to take longer than in tropical carbonates, with lithification herein not beginning until similar to 200 ka and complete cementation being achieved in similar to 450 ky Thus, neither aeolianites nor offshore marine carbonates record the composition of the original sediment; aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks. This partitioning, revealed for the first time, answers the conundrum as to why some cool-water neritic shelf and coastal sediments in the rock record are compositionally different.