Year

2012

Degree Name

Doctor of Philosophy

Department

School of Earth & Environmental Sciences

Abstract

Estimates on the extent of time-averaging and age-mixing in sediments from semi-enclosed shallow marine systems (deltas, aggrading clastic shelves, and carbonate platforms) are sparse. Yet, semi-enclosed basins are particularly important for palaeoclimatic studies, because they commonly retain proxy evidence related to oscillations in the level of the sea, at both local (relative) and global (eustatic) scales. However, an accurate and realistic chronology of semi-enclosed basins can be especially difficult to formulate due to the repeated oscillation of sea level, the consequent inundation by and regression of the sea, and the invariably repetitive and alternating cycles of prompt growth and death of populations of marine, brackish and non-marine assemblages upon which dating techniques are commonly applied. Thus, this may result in punctuated stratigraphies, significant hiatuses in the chronological record, particularly because of the mixing of non-, marginal- and fully marine sediments during transitions from one hydrodynamic regime to another.

This project investigates the extent of time-averaging in three semi-enclosed basins, the Gulf of Carpentaria, and Gulf St Vincent, Australia; and the Black Sea. These basins vary in scale and depth, climate, and the extent to which they are open to the ocean, the latter being particularly influenced by the depth of the entrance or sill. Amino acid racemization, accelerator mass spectrometry 14C and uranium-series dating were utilised to estimate the degree to which mixed-age fossil assemblages contribute to the sedimentary record in Gulf St Vincent basin, at Kingscote, Kangaroo Island, South Australia, at Karumba, Queensland, and the central Gulf of Carpentaria, Northern Australia, and at Kerch Strait, Northeastern Black Sea, the Danube Delta coast, and the Northwest shelf of the Black Sea.

The extent of time-averaging in Holocene sediments from these basins was determined to be: Gulf St Vincent – 30-40 ka; Gulf of Carpentaria, ~ 10-12 ka; the Black Sea shelves, 5-6 ka. Broadly speaking, these results indicate that in large shallow basins moderately open to the influence of the global ocean (here, the Gulf of Carpentaria), reworking of sediment is an issue for chronology, and perhaps to a greater extent for defining palaeo-environments, because sediments and fossils from several different niches may regularly contribute to each sedimentary stratum, and thus though the extent of time-averaging may not be significant in numbers of years, the presence of mixed-source sediments can be problematic for defining a reliable chronology. Small fluctuations in water-level about the level of the enclosing sill can result in the inundation and/or exposure and subsequent reworking of material for dating from unrelated environments.

In smaller shallow estuarine basins (here Gulf St Vincent), where the sill has less influence on the volume of water held within the basin, the ingress and regress of marine water results in stratigraphies that are punctuated. In contrast, large semi-enclosed basins whose interaction with the marine realm is strongly controlled by the sill or entrance (the Black Sea) typically contain punctuated stratigraphies that are easily discernable in broad shelf settings, similar to Gulf St Vincent, but also commonly have on their coastal margins strata that are mixed.

The scale of time-averaging in these and similar basins is a function of the availability, and preservation of fossils as source material. In the samples examined, the scale of time-averaging for the Gulf of Carpentaria may be up to 12 ka and indicated by the presence of early Holocene non-marine shells in surface sediments from the central basin; in Gulf St Vincent, the extent of time-averaging in sediments recovered from the central basin is up to 30-40 ka, indicated by the presence of individually dated MIS 3 age Elphidium in Holocene sediments; whereas in the Black Sea the extent of time-averaging in Holocene shelf environments is comparatively small, being only 5-6 ka. The principal difference in the sedimentary record among these basins is that little sediment is removed from the two shallow Australian marginal marine basins, whereas in contrast, a large proportion of sediment originating from coastal environments and deposited on the shelves of the Black Sea is transported ultimately from the shelves to the continental slopes and distal deep basin.

These results indicate that the chronological recognition of reworked fossils at high resolution is a requisite for determining accurate palaeoenvironments and ages, especially over Holocene time. Given the capability of modern analytical instruments, it is insufficient, especially in young marginal marine sediments which may be statigraphically and chronologically mixed, to estimate an age for a population of fossils based on the assemblage alone, or on bulk samples from an assemblage. Modern dating methods must be utilised in determining the age of individual bioclasts in sediments prior to associating an environment to the sediments being studied because chronological and palaeoenvironmental estimates based on bulk samples or assemblages have a large likelihood of being unreliable.

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