Sediment color variation in laminated Holocene sediments of ODP Site 178-1098

Continuous sediment color records with a resolution of one measurement per millimeter were generated for Site 1098 (Palmer Deep, Antarctic Peninsula) from digital images of the core surfaces to test if the laminated intervals at this site will allow for analysis of high-frequency climate variability in the Circum-Antarctic. Long-term variation in color values correlates with gamma-ray attenuation bulk density. Darker colors are found in laminated intervals with lower bulk density, high biogenic silica, and high total organic carbon content. Darker color values result from the addition of dark laminae to background sediments that show little variation in color. The thicknesses of dark and light laminae were measured in the top 25 meters composite depth to determine the temporal resolution of the laminae. The alternation between dark, biogenic-rich laminae and background sediment essentially represents an annual cycle, but the sediment is not consistently varved. The modal thickness of light laminae is close to the long-term average annual accumulation rate, and results indicate that approximately half of the dark/light couplets in distinctly laminated intervals represent a single year. Missing biogenic laminae are interpreted to represent reduced primary productivity during cold years with delayed breakup of the sea-ice cover.

C4-C7 alkenones in deep sea sediments

It is believed that C4 to C7 hydrocarbons in petroleum are formed by the cracking of organic matter at depths generally exceeding 1,000 m at temperatures in excess of 50 °C (Cordel, 1972; Dow, 1974; Tissot et al., 1974)). Also, none of the alkanes in the butane-heptane range are formed biologically as far as is known at present. Consequently, it is thought that they do not occur in shallow, Recent sediments. In 1962, I analysed 22 samples of Recent sediments from 7 different environments and verified that these hydrocarbons were not present at the p.p.m. level (Dunton and Hunt, 1962) although traces of a few hydrocarbons such as butane, isobutane, isopentane and n-heptane have been found (Sokolov, 1957; Veber and Turkeltaub, 1958; Erdman et al., 1958; Emery and Hoggan, 1958). No identification of individual hexanes or heptanes has been reported except when there has been clear evidence of seepage from deeper source sediments (McIver, 1973).

Benthic microalgae in coral reef sediments of the southern Great Barrier Reef, Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m(-2)), and productive (up to 110 mg O-2 m(-2) h(-1)) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O-2 evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92-995 mg chl a m(-2)) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.

Top down control in a relatively pristine seagrass ecosystem

The loss of large-bodied herbivores and/or top predators has been associated with large-scale changes in terrestrial, freshwater, and marine ecosystems around the world. Understanding the consequences of these declines has been hampered by a lack of studies in relatively pristine systems. To fill this gap, I investigated the dynamics of the relatively pristine seagrass ecosystem of Shark Bay, Australia. I began by examining the seagrass species distributions, stoichiometry, and patterns of nutrient limitation across the whole of Shark Bay. Large areas were N-limited, P-limited, or limited by factors other than nutrients. Phosphorus-limitation was centered in areas of restricted water exchange with the ocean. Nutrient content of seagrasses varied seasonally, but the strength of seasonal responses were species-specific. Using a cafeteria-style experiment, I found that fast-growing seagrass species, which had higher nutrient content experienced higher rates of herbivory than slow-growing species that are dominant in the bay but have low nutrient content. Although removal rates correlated well with nutrient content at a broad scale, within fast-growing species removal rates were not closely tied to N or P content. Using a combination of stable isotope analysis and animal borne video, I found that green turtles (Chelonia mydas)—one of the most abundant large-bodied herbivores in Shark Bay—appear to assimilate little energy from seagrasses at the population level. There was, however, evidence of individual specialization in turtle diets with some individuals foraging largely on seagrasses and others feeding primarily on macroalgae and gelatinous macroplankton. Finally, I used exclusion cages, to examine whether predation-sensitive habitat shifts by megagrazers (green turtles, dugongs) transmitted a behavior-mediated trophic cascade (BMTC) between sharks and seagrasses. In general, data were consistent with predictions of a behavior-mediated trophic cascade. Megaherbivore impacts on seagrasses were large only in the microhabitat where megaherbivores congregate to reduce predation risk. My study highlights the importance of large herbivores in structuring seagrass communities and, more generally, suggests that roving top predators likely are important in structuring communities—and possibly ecosystems—through non-consumptive pathways.

Stable isotope and Cd/Ca ratios of Neogloboquadrina pachyderma in high-latitude sediments

Recent studies have stressed the role of high latitude nutrient levels and productivity in controlling the carbon isotopic composition of the deep sea and the CO2 content of the atmosphere. We undertook a study of the chemical composition of the polar planktonic foraminifer Neogloboquadrina pachyderma (s., sinistral coiling) from 30 late Holocene samples and 49 down core records from the high-latitude North and South Atlantic Oceans to evaluate the history of sea surface chemical change from glacial to interglacial time. Stable isotopic analysis of coretop samples from the Atlantic, Pacific and Southern Oceans shows no significant correlation between the delta13C of N. pachyderma and either delta13C or PO4 in seawater. Conversely, Cd/Ca ratios in planktonic foraminifera are consistent with the PO4 content of surface waters. The level of maximum glaciation (18,000 yr B.P.), identified by CLIMAP and delta18O, was chosen for mapping. Isopleths of delta18O on N. pachyderma (s.) in the North Atlantic reveal a pattern largely influenced by sea surface temperature (S.S.T.) and generally support the S.S.T. reconstruction of CLIMAP. Differences between the two suggest significantly lower salinity in North Atlantic surface waters at high latitudes than in lower latitudes. Down core delta13C records of N. pachyderma confirm that low delta13C values occurred in the northeast Atlantic during the latest glacial maximum (Labeyrie and Duplessy, 1985, doi:10.1016/0031-0182(85)90069-0). However, a map of delta13C for the 18,000 yr B.P. level for a much larger region in the North Atlantic shows that minimum N. pachyderma delta13C occurred in temperate waters. N. pachyderma delta13C decreased toward the southwest, reaching a minimum of -1 per mil at 37°N. Despite the variability seen in delta13C records of N. pachyderma, none of our cores show significant temporal variability in Cd/Ca. From the combined Cd/Ca and delta13C data we can see no evidence for an upwelling gyre in the eastern North Atlantic during the latest glacial maximum, nor evidence that the southern and northern oceans had significantly different levels of preformed nutrients than today.

(Table 1) Planktonic foraminifera counts and stable oxygen isotope ratios of Pacific Ocean surface sediments

Core-top samples from the eastern tropical Pacific (10°N to 20°S) were used to test whether the ratio between Globorotalia menardii cultrata and Neogloboquadrina dutertrei abundance (Rc/d) and the oxygen isotope composition (?18O) of planktonic foraminifera can be used as proxies for the latitudinal position of the Equatorial Front. Specifically, this study compares the ?18O values of eight species of planktonic foraminifera (Globigerinoides ruber sensu stricto (ss) and sensu lato (sl), Globigerinoides sacculifer, Globigerinoides triloba, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Globorotalia menardii menardii, Globorotalia menardii cultrata and Globorotalia tumida) with the seasonal hydrography of the region, and evaluates the application of each species or combination of species for paleoceanographic reconstructions. The results are consistent with sea surface temperature and water column stratification patterns. We found that in samples north of 1°N, the Rc/d values tend to be higher and d18O values of G. ruber, G. sacculifer, G. triloba, P. obliquiloculata, N. dutertrei, and G. menardii cultrata tend to be lower than those from samples located south of 1°N. We suggest that the combined use of Rc/d and the d18O difference between G. ruber and G. tumida or between P. obliquiloculata and G. tumida are the most suitable tools for reconstructing changes in the latitudinal position of the Equatorial Front and changes in the thermal stratification of the upper water column in the eastern tropical Pacific.