Calcareous epiphyte production in cool-water carbonate seagrass depositional environments – southern Australia - Perspectives in Carbonate Geology - page 121

Calcareous epiphyte production in cool-water carbonate seagrass
depositional environments - southern Australia

NOEL P. JAMES*, YVONNE BONE ‡ , KIRSTY M. BROWN †‡ and
ANTHONY CHESHIRE ‡
* Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada
K7L 3N6 (E-mail: [e-mail])
‡ School of Earth and Environmental Sciences, University of Adelaide, South Australia, Australia 2005
(E-mail: [e-mail])

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 epi-
phytes 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 (articu-
lated) and non-geniculate (encrusting) coralline algae that together comprise ~38-80%
of the epiphyte carbonate. The only other signifi cant epiphytes are bryozoans and ben-
thic foraminifera, which contribute roughly equal amounts (~8-33% each) of carbon-
ate. 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 epi-
phytic carbonate are calculated to be ~7.4 cm kyr 1 . This accounts for a major propor-
tion 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 near-
shore, 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.

Keywords Cool-water carbonate, epiphyte, southern Australia, seagrass, sedimentation,
coralline algae.

INTRODUCTION guided through the spectacular shallow-water

environments of the Caribbean by Robert Ginsburg
cannot help, egged on by his enthusiasm, but be
surprised by the abundance and diversity of
the calcareous benthos and the ubiquity of the
marine grasses. The lush meadows seem to be a
hallmark of the brightly lit, warm, shallow, marine
carbonate seafl oor. In his seminal studies of car-
bonate sedimentology from the Florida-Caribbean
region, Ginsburg has repeatedly stressed the
importance of these plants to the depositional sys-
tem (Ginsburg, 1956; Nelsen & Ginsburg, 1986).
Their function as sediment binders, as refuges
for infaunal and epifaunal, mobile and sessile

The nature of a carbonate sedimentary rock refl ects
the environment in which it formed and the time
in which it was generated. This axiom, as articu-
lated by Robert Ginsburg, is one of the founding
principles of comparative sedimentology. The
modern seafl oor is a prism through which the
ancient rock record is viewed, and so the better
modern sedimentary processes are understood, the
more confi dent is our interpretation of the ancient
sedimentary rock record. Anyone who has been

† Deceased.