Jurassic to Cretaceous calcareous nannofossils from the Argo Abyssal Plain

A summary of calcareous nannofossil biostratigraphy performed for Late Jurassic (Kimmeridgian) to Early Cretaceous (Hauterivian) cores of Site 765 (Cores 123-765C-58R to -55R) and Site 261 (Cores 27-261-33 to -27), Argo Abyssal Plain, off northwestern Australia is presented. Precise age determinations were limited by variable preservation and the exclusion of a number of marker species due to provincialism. However, the presence of species, such as, Stephanolithion bigotii bigotii, Watznaueria manivitae, Tubodiscus verenae, and Cruciellipsis cuvillieri results in a reasonably good degree of biostratigraphic control. Biogeographic interpretation of the nannofossil data suggests that the Argo Basin occupied a position transitional between the Tethyan and Austral nannofloral realms. A cooler water regime is suggested by the absence of thermophyllic Tethyan forms, such as Nannoconus, and the presence of taxa that display bipolar distribution, such as Crucibiscutum salebrosum. Two new species, Zeugrhabdotus cooperi and Cyclagelosphaera argoensis, and one new combination, Haqius ellipticus are described.

Paleomagnetic of lower Cretaceous sediments from the Argo and Gascoyne Abyssal Plain

Lower Cretaceous sediments were sampled for magnetostratigraphy at three sites. ODP Site 765 and DSDP Site 261, in the Argo Abyssal Plain, consist primarily of brownish-red to gray claystone having hematite and magnetite carriers of characteristic magnetization. ODP Site 766, in the Gascoyne Abyssal Plain, consists mainly of dark greenish-gray volcaniclastic turbidites with magnetite as the carrier of characteristic magnetization. Progressive thermal demagnetization (Sites 765 and 261) or alternating field demagnetization (Site 766) yielded well-defined polarity zones and a set of reliable paleolatitudes. Magnetic polarity chrons were assigned to polarity zones using biostratigraphic correlations. Late Aptian chron M"-1"r, a brief reversed-polarity chron younger than MOr, is a narrow, 40-cm feature delimited in Hole 765C. Early Aptian reversed-polarity chron MOr is also present in Hole 765C. Polarity chrons Mir through M3r were observed in the Barremian of all three sites. Valanginian and Hauterivian polarity chrons can be tentatively assigned to polarity zones only in Hole 766A. The paleolatitude of this region remained at 35° to 37°S from the Berriasian through the Aptian. During this interval, there was approximately 16° of clockwise rotation, with the oriented sample suites of Site 765 displaying a Berriasian declination of 307° to an Aptian declination of 323°. These results are consistent with the interpolated Early Cretaceous apparent polar wander for Australia, but indicate that this region was approximately 5? farther north than predicted.

Geochemistry of Cretaceous sediments from Argo and Gascoyne Abyssal Plains

The accumulation of organic matter, ferrous and pyrite iron, and the ratios of organic carbon/total sulfur and organic carbon/total phosphorus in the Lower Cretaceous sediments from the Argo and Gascoyne abyssal plains have been used as indicators of both the source and reactivity of organic matter in the sediments and the depositional environment. Total sulfur, used as an indicator of pyrite sulfur, is more abundant in sediments from the Gascoyne Abyssal Plain than in those from the Argo Abyssal Plain. Sulfur positively correlates with TOC at both sites (although poorly at the Argo Abyssal Plain site, R = 0.48), with an extension of the line of best-fit through the origin, indicating that pyrite (TOC <2 wt%) is diagenetic and deposited from normal marine conditions. The average ratio of C/S for samples of TOC <2 wt% is 5.4 at Argo Abyssal Plain (compared to the modern normal marine value of 2.8) indicating deposition of organic matter probably of mixed terrestrial and oxidized marine sources that is unreactive to the sulfate-reducing bacteria. One sample from the Aptian sediments is rich in TOC (5.1 wt%) and has a C/S ratio of 0.5. The average C/S ratio in Gascoyne Abyssal Plain sediments is 0.8 (R = 0.97), which indicates the formation of abundant pyrite in addition to burial and preservation of relatively fresh organic matter that is reactive to the sulfate-reducing bacteria. Organic carbon to phosphorus ratios (C/P) in the sediments indicate preferential remobilization of organic carbon over phosphorus with increasing water depth. Estimates of the degree of pyritization (DOP) increase with increasing TOC at both sites, indicating iron is not limiting and pyrite is formed diagenetically. The one sample with a TOC content of 5.1 wt%, from the Argo Abyssal Plain near the Barremian-Aptian boundary, is composed mostly of framboidal pyrite, finely laminated and not bioturbated, and hence may have been deposited during a brief period of anoxia in the overlying waters.

Objective Mapping of Argo data in the Weddell Gyre: a gridded dataset of upper ocean water properties, link to data files in NetCDF format

The Weddell Gyre plays a crucial role in the regulation of climate by transferring heat into the deep ocean through deep and bottom water mass formation. However, our understanding of Weddell Gyre water mass properties is limited to regions of data availability, primarily along the Prime Meridian. The aim is to provide a dataset of the upper water column properties of the entire Weddell Gyre. Objective mapping was applied to Argo float data in order to produce spatially gridded, time composite maps of temperature and salinity for fixed pressure levels ranging from 50 to 2000 dbar, as well as temperature, salinity and pressure at the level of the sub-surface temperature maximum. While the data are currently too limited to incorporate time into the gridded structure, the data are extensive enough to produce maps of the entire region across three time composite periods (2002-2005, 2006-2009 and 2010-2013), which can be used to determine how representative conclusions drawn from data collected along general RV transect lines are on a gyre scale perspective. The time composite data sets are provided as netCDF files; one for each time period. Mapped fields of conservative temperature, absolute salinity and potential density are provided for 41 vertical pressure levels. The above variables as well as pressure are provided at the level of the sub-surface temperature maximum. Corresponding mapping errors are also included in the netCDF files. Further details are provided in the global attributes, such as the unit variables and structure of the corresponding data array (i.e. latitude x longitude x vertical pressure level). In addition, all files ending in "_potTpSal" provide mapped fields of potential temperature and practical salinity.