Outer Darwin Harbour Marine Survey 2015: Total sediment metabolism, mineral specific surface area, carbonate and element concentrations and C and N isotopes in seabed sediments.

Brief description

This resource contains surface sediment data for Outer Darwin Harbour collected by Geoscience Australia (GA), the Australian Institute of Marine Science (AIMS) and the Northern Territory Government (Department of Land Resource Management) during the period from 28 May and 23 June 2015 on the RV Solander (survey SOL6187/GA0351). This project was made possible through offset funds provided by INPEX-led Ichthys LNG Project to Northern Territory Government Department of Land Resource Management, and co-investment from Geoscience Australia and Australian Institute of Marine Science. The intent of this four year (2014-2018) program is to improve knowledge of the marine environments in the Darwin and Bynoe Harbour regions by collating and collecting baseline data that enable the creation of thematic habitat maps that underpin marine resource management decisions. The specific objectives of the survey were to:

1. Obtain high resolution geophysical (bathymetry) data for outer Darwin Harbour, including Shoal Bay;
2. Characterise substrates (acoustic backscatter properties, grainsize, sediment chemistry) for outer Darwin Harbour, including Shoal Bay; and
3. Collect tidal data for the survey area.

Data acquired during the survey included: multibeam sonar bathymetry and acoustic backscatter; physical samples of seabed sediments, underwater photography and video of grab sample locations and oceanographic information including tidal data and sound velocity profiles. These datasets comprise total sediment metabolism, mineral specific surface area and carbonate and element concetrations, and C and N isotopes of seabed sediments.

A detailed account of the survey is provided in: Siwabessy, P.J.W., Smit, N., Atkinson, I., Dando, N., Harries, S., Howard, F.J.F., Li, J., Nicholas, W.A., Potter, A., Radke, L.C., Tran, M., Williams, D. and Whiteway, T., 2015. Outer Darwin Harbour Marine Survey 2015: GA0351/SOL6187 Post-survey report. Record 2016/008. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2016.008

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Bottom sediments were collected using a Smith McIntyre Grab. The surface sediments (0-2 cm) within each grab were spooned into falcon vials and the pore waters were removed by centrifugation. Pore waters were removed within 20 minutes of collection, and salinity, temperature and pH measurements were taken. The pore waters were then filtered (0.45 µm) into 3 ml gas-tight vials (that were pre-charged with 0.025 HgCl2). The procedure was repeated on pore waters from a second bulk sample that was incubated for ~24 hrs at sea surface temperatures. The samples were then frozen for transport to the laboratories at Geoscience Australia (GA) where they were: (1) subsampled for specific surface area analysis; and (2) freeze-dried and ground in a tungsten carbide mill. The dried residue was divided into fractions for: (i) major and trace element analysis; (ii) bulk carbonate analyses; and (ii) total organic carbon and total nitrogen concentrations and isotopic ratios (after de-carbonation).

Dissolved inorganic carbon (DIC) concentrations were determined on pore waters using a DIC analyser and infrared-based CO2 detector (Geoscience Australia). CO2 production rates were calculated by concentration differences over the incubation period, after correction for CaCO3 fluxes. The average %RSD of the precisions and accuracies of the dissolved inorganic carbon measurements were 0.2. The accuracy of the wet/dry weight used in the calculations were better than 1%.

Bulk carbonate was determined on dried sediment using the carbonate bomb method. Specific surface areas were determined using a 5-point Brunauer-Emmett-Teller (BET) adsorption isotherm on a Quantachrome NOVA 2200e analyser, with nitrogen used as the adsorbate. The samples were first cleaned of organic matter by slow heating for 12 hours to 350oC. Average %RSD plus/minus the standard deviation of 16 samples run in duplicate was 5.9 +/- 9.0%. The accuracy of the method is +/-3.2%.

Major and trace elements were determined by X-Ray fluorescence at GA using a Phillips PW204 4kW sequential spectrometer. A more comprehensive suite of trace elements was determined using an AGILENT 7500ce ICP-MS, also at GA. For the XRF, Standard Reference Materials (SRM) called CH-1 (Marine Sediment; Institute of Rock and Mineral Analysis, Beijing) and W6-1 were was used to calculate accuracy. Deviation of measured values from expected values (across the elements) ranged from 0-5.6% (CH-1) and 0-66.9% (W6-1). Twelve samples were run in duplicate to measure precision, and the average % RSD across the elements ranged from 0-32%. The accuracy and precision information is presented on an element by element basis in accompanying spreadsheet.

For the ICP-MS analyses, three different SRM's were used to calculate accuracy: CH-1 (Marine Sediment; Institute of Rock and Mineral Analysis, Beijing), W6-1 and sietronics. The average deviation of measured values from expected values (across the elements) were 6.8% (CH-1; n=6.8), 2.2% (W6-1; n=11) and -2.5% (sietroncs; n=9). Ten sediment samples were run in duplicate to measure precision. The average %RSD across the elements and the 10 sets of duplicates was 8 +/- 5.5% and thus was quite variable. The accuracy and precision information is presented on an element by element basis in accompanying spreadsheet together with output for quartz blanks and milling precision.

De-carbonated powders were sent to Environmental Isotopes Pty Ltd (Sydney) for isotopic analysis by mass spectrometry. Samples were back-corrected to account for the carbonate removal, using carbonate concentrations derived from the bomb method (this dataset). Error estimates for the C and N isotope values are ±0.15.

Thanks to the crew of the RV Solander for help with sample collection.