Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf, South Australia

Abstract

Spencer Gulf is a large (ca 22 000 km2), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to −28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m−3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m−3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m−3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec−1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.

Heterozoan carbonates: When, where and why? A synthesis on parameters controlling carbonate production and occurrences

“…above cited references for a discussion about the Cenozoic and (Bosence, 1983;Bosence & Pedley, 1982;Freiwald, 1998), pp. 772-773; (Hallock, 2001), p. 407; (Henrich et al, 1995;O'Connell et al, 2016), for a discussion about red algal controlling parameters).…”

“…Kelp forests form a specific carbonate factory within hard-bottom areas within high hydrodynamic settings (Freiwald, 1998;James et al, 2013). (2) Oligo-mesotrophic, subtropical or warm-temperate seas (typically the Mediterranean Sea) in which the characteristic carbonate sediments are red algae and seagrass-derived bioclastic grains (Betzler et al, 1997;Fornos & Ahr, 1997;James et al, 2009a;O'Connell et al, 2016;Perry & Beavington-Penney, 2005). The warm-temperate carbonates are controlled by light penetration that allows for photosynthesis (seagrass and algae; (Bosence, 1985), about coralline algae), and thus has the same primary driver as photozoan carbonates.…”

“…As revealed by the fact that the occurrence of extensive warm-temperate carbonates are restricted to semi-enclosed basins or gulfs (i.e. the Mediterranean Sea and South Australian gulfs; (Betzler et al, 1997;James & Bone, 2011;O'Connell et al, 2016)), hydrodynamics should have an intermediate strength, high enough to winnow terrigenous input and fines away from the depositional realm ((Birkett et al, 1998), p. 25), but not too much to develop extensive deposits ((Betzler et al, 1997;Bosence, 1985;Henrich et al, 1995); Hottinger, 1983). By contrast, red algal ridges and crusts, which are resistant to high hydraulic regimes and are part of the coral reef ecosystem (cf.…”

Capturing expert uncertainty in spatial cumulative impact assessments

“…In addition, the available climate change data had poor spatial resolution, meaning that these threats were often treated as ubiquitous (Supplementary Figure S3e ). We were not able to include three particularly vulnerable ecosystems (rhodolith beds 31 , 32 , sponge gardens 30 , 64 and native shellfish beds 33 ) because of a lack of spatial data leading to uncertainty around their location and spatial extent throughout the Gulf. The unavailability of these data identify clear knowledge gaps based around a lack of well-resolved spatial information on ecosystems, human activities and impacts of climate change, as well as a need for more empirical studies on the impact of various threats, including the likely complex interactions between multiple threats 65 .…”

Evolution of a carbonate delta generated by gateway‐funnelling of episodic currents

“…Cool‐water carbonate production (Lees & Buller, ; Schlager, ) is controlled by heterozoan organisms (James, ), such as bivalves, echinoids, bryozoans, benthic foraminifera and coralline red algae (O'Connell et al ., ) which live in water depth of tens to hundreds of metres in the present‐day Mediterranean (Fornós & Ahr, ). Due to weak early cementation, heterozoan skeletal debris on the sea floor remains unconsolidated (James & Bone, ) which, in combination with the absence of reef barriers, facilitates sediment mobilization during storms and occasional currents (Pedley & Carannante, ), for example in tidal straits (Anastas et al ., ; Longhitano, ) and marine corridors (Betzler et al ., ; Puga‐Bernabéu et al ., ).…”