Calcareous Epiphyte Production in Cool‐Water Carbonate Seagrass Depositional Environments — Southern Australia

Abstract

The southern continental margin of Australia, the largest area of cool-water carbonate sedimentation on the globe, is characterized by extensive marine grassbeds in many inshore environments. The most important seagrasses in terms of calcareous epiphyte production are Posidonia sinuosa, P. angustifolia, P. australis, Amphibolis antarctica and A. griffi thii. The predominant control on relative abundance of calcareous epiphytes is seagrass biomass. These grasses have a biomass of 50-500 g m 2 , which peaks at 2-4 m water depth. The most abundant calcareous epiphytes are geniculate (articulated) and non-geniculate (encrusting) coralline algae that together comprise ~38-80% of the epiphyte carbonate. The only other signifi cant epiphytes are bryozoans and benthic foraminifera, which contribute roughly equal amounts (~8-33% each) of carbonate. Unlike the seagrass biomass, calcareous epiphyte abundance peaks at water depths of ~10 m. The rates of epiphyte production are roughly similar to those from epiphytes in tropical environments, averaging 210 26 g m 2 yr 1 . Posidonia is morphologically similar to tropical seagrasses (e.g. Thalassia) and produces largely carbonate mud from the disintegration of blade-encrusting corallines. Amphibolis, on the other hand, has an extensive upright, exposed shoot system that is much longer lived (biennial) and so is encrusted with prolifi c articulated corallines thus producing ~3 more carbonate in terms of g kg 1 from the stems than the blades. Average accumulation rates of epiphytic carbonate are calculated to be ~7.4 cm kyr 1 . This accounts for a major proportion of the carbonate sequestered in grass beds in this cool-water realm, and probably accounts for much of the nearshore and supratidal carbonate mud. Thus, the nearshore, grass-covered habitat is a cool-water carbonate factory surprisingly similar to the shallow-water tropical system, except that the sediment produced is poorly sorted Mg-calcite carbonate with little or no aragonite.

Quaternary aeolianites in south‐east Australia – a conceptual linkage between marine source and terrestrial deposition

“…(Fig. 18) a benthic foraminifer that is commonly attached to seagrass blades James et al, 2009). Bryozoans are comparatively few and almost absent in the Dairy Range, supporting the contention that little sediment came from the mid-shelf.…”

“…Seagrass meadows, amongst the most prolific in the modern world (Edgar, 2001), have epiphytic, epifaunal and infaunal calcareous algae and invertebrates living on the grass blades and stems as well as between the plants and in the sediment between rhizomes . Sediment produced in these marine meadows is dominated by geniculate coralline algae, benthic foraminifera and mollusc fragments Cann et al, 2002;James et al, 2009). The seagrass, while providing a substrate for epiphytic algae and invertebrates, also fixes sediment in place via rhyzomes that prevent erosion and transport.…”

Carbonate sediments in a cool‐water macroalgal environment, Kaikoura, New Zealand

“…1), support small patches of Zostera. Epiphytes were not apparent on Zostera blades, in contrast to the warm temperate settings of South Australia (James et al, 2009) and other locations (Wanless, 1981;Perry & Beavington-Penney, 2005).…”

Pleistocene aeolianites at Cape Spencer, South Australia; record of a vanished inner neritic cool‐water carbonate factory

“…(whelk), Mitrella sp., Nerita atramentosa and Amalda monilifera . This assemblage lives today in sheltered areas of rocky coasts, under rocks, on offshore reefs and in intertidal to shallow seagrass or macroalgal environments, but always <10 m water depth (Edgar, ; James et al ., , ; Ludbrook, ). Analysis of a Sanguinolaria (Psammotellina) biradiata shell and a Turbo sp.…”

“…The purpose of this study is: (i) to ascertain whether different parts of the succession record changing offshore marine carbonate factories and thus varying temporal palaeoceanography; and (ii) to determine how many sea‐level cycles are represented in this 80 m thick series of stacked palaeodunes. This type of analysis is now possible not only because of recent detailed analysis of modern inner neritic environments but also of open ocean seagrass meadows (James et al ., ) and macroalgal forests (James et al ., ).…”