View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NERC Open Research Archive National Oceanography Centre, Southampton Research & Consultancy Report No. 71 Preparation of a bathymetric map and GIS of the South Atlantic Ocean and a review of available biologically relevant South Atlantic Seamount data for the SEAFO Scientific Committee C L Jacobs and B J Bett 2010 National Oceanography Centre, Southampton University of Southampton Waterfront Campus European Way Southampton Hants SO14 3ZH UK Author contact details: Tel: +44 (0)23 8059 6576 Fax: +44 (0)23 8059 6554 Email: [email protected] DOCUMENT DATA SHEET AUTHOR PUBLICATION DATE JACOBS, C L and BETT, B J 2010 TITLE Preparation of a bathymetric map and GIS of the South Atlantic Ocean and a review of available biologically relevant South Atlantic Seamount data for the SEAFO Scientific Committee REFERENCE Southampton, UK: National Oceanography Centre, Southampton, 18p. (National Oceanography Centre Southampton Research and Consultancy Report, No. 71) (Unpublished manuscript) ABSTRACT This report has brought together various data sets from a number of public sources around the world to produce the most up-to-date regional bathymetric map of the South Atlantic in both printed and digital form. The use of a data control layer in the GIS allows users of this report an indication of the data accuracy and quality. Physical and chemical parameters have also been included in the GIS, so that the Temperature, Salinity and Oxygen content of the South Atlantic, all primary agents in the distribution of the various forms of biology can be examined, at different critical depths at the same time. These critical depths were determined to be those that had most effect on the biology, and were the sea surface (0m), the lower limit of the photic zone, which we approximated to 200m, and the lower limit of the likely vertical migration of zooplankton, a major source of food at 1000m depth. Other elements presented include biogeographical provinces defined by Longhurst, a geographic depiction of the formally UN- backed named Seamount gazetteer along with a further seamount-science web-based product sponsored by the National Science Foundation in the USA, and a biological catalogue of data supported by the Census of Marine Life. KEYWORDS South Atlantic seamounts bathymetry salinity dissolved oxygen sea temperature ISSUING ORGANISATION National Oceanography Centre, Southampton University of Southampton, Waterfront Campus European Way Southampton SO14 3ZH UK ii This page intentionally left blank iii Executive Summary This report discusses the data used in the production of the map and GIS layers, and also briefly outlines and discusses each type of delivered data layer. It has brought together various data sets from a number of public sources around the world to produce the most up-to-date regional bathymetric map of the South Atlantic in both printed and digital form, along with an interactive 3D view of the same data. This will allow not only an appreciation of the gross bathymetric features of the region, but the use of a data control layer in the GIS will allow users of this data an indication of the data accuracy and quality. Physical and chemical parameters have also been included in the GIS, so that the Temperature, Salinity and Oxygen content of the South Atlantic, all primary agents in the distribution of the various forms of biology can be examined, at different critical depths at the same time. These critical depths were determined to be those that had most effect on the biology, and were the sea surface (0m), the lower limit of the photic zone, which we approximated to 200m, and the lower limit of the likely vertical migration of zooplankton, a major source of food at 1000m depth. Other elements presented include biogeographical provinces defined by Longhurst, a geographic depiction of the formally UN-backed named Seamount gazetteer along with a further seamount-science web-based product sponsored by the National Science Foundation in the USA, and a biological catalogue of data supported by the Census of Marine Life. These final elements of this report suggest that data on South Atlantic seamounts, especially in terms of biologically-significant data is at best described as very patchy and of variable quality. 1 Table of Contents Executive Summary.........................................................................................1 Table of Contents.............................................................................................2 Bathymetry Grid ....................................................................................3 Printed Map...........................................................................................4 ArcGIS Data..........................................................................................4 3D Data.................................................................................................9 Seamount Biological Data...................................................................10 Discussion...........................................................................................11 Recommendation ................................................................................13 Figure List ......................................................................................................14 List of Acronyms ............................................................................................15 References ....................................................................................................16 Additional Notes (for GIS use) .......................................................................18 2 Bathymetry Grid The bathymetry compilations (map, GIS and 3D view) have used the very latest (February 2010) version of the GEBCO_08 global bathymetric dataset (www.gebco.net). The global bathymetric grid was generated by combining over 290 million quality-controlled ship depth soundings (single and multibeam) with interpolation between sounding points based upon version V16.1 (created in March 2007) of the Sandwell and Smith gravity anomaly from Geosat and ERS1 satellite altimetry (Sandwell and Smith 1997). The grid used here is a sub-set of that global grid. The GEBCO global bathymetry grid was originally derived from the SRTM30_plus grid released by Becker et al in 2004. Version 5.0 of the Becker et al’s SRTM30_plus grid, released in September 2008, forms the basis of the GEBCO_08 grid, which itself was first released in January 2009 and updated in November 2009. The latest version of the GEBCO_08 grid has undergone six iterations of identifying and removing apparent “bad soundings” and construction of updated grids. However the GEBCO_08 grid is a development product and will be periodically updated as more artefacts are identified and new, good-quality data added. It should be noted that there appear to be some “bad” data tracks along the continental margin off South Africa which give rise to a highly convoluted 200 m depth contours. It was beyond the scope of this report to remove and/or edit such data. The bathymetry gird is essentially as series of “tiles” that contain either a real or predicted depth value. Where real depth values are used, it may be that there is one or more actual sounding within that tile, whereupon the depth value for that tile will be derived from the median value of all the depths contained within that tile. To enable identification of what tiles are controlled and which contain predicted depths, a ‘source identifier’ grid has also been made available in the GIS, labelled as “Data Control”. The GEBCO_08 grid has a spatial resolution of 30 arc-seconds (so each data tile is 30 arc-seconds in length and width, approx 900m), and the datum is nominally mean sea level. 3 The land elevation data is from the US space shuttle SRTM mission, and as it is not a primary requirement of the report is not discussed further here. Further information if required can be obtained from the references and list if Acronyms. Printed Map The printed map (Figure 1) supplied shows the primary interest features, namely the bathymetry, colour coded with depth, with as highlighted contours the approximate lower limit of the photic zone (200 m, contoured in orange) and the likely vertical limit of zooplankton migration (1000 m, contoured in red). Figure 1. The bathymetry of the South Atlantic Ocean colour-coded by depth. Also highlighted here are areas of probable erroneous depths (contour irregularities). ArcGIS Data The ArcGIS data files include the bathymetry (as seen above and in the separate printed plots), along with a number of other parameters that either help understand the underlying bathymetry data, or have a direct relevance to 4 the biophysical parameters that occur in the South Atlantic Ocean. These are all listed and briefly described below. The ArcGIS data have been assembled using the latest version of ESRI ArcMap™ and ArcCatalog™ – version 9.3 (build 1770). They can be displayed using either of the ESRI ArcMap Documents SEAFO_Arc93.mxd1 or SEAFO_Arc92.mxd2 dependant upon whether the user is using ArcMap 9.22 or ArcMap 9.31. All data and derivatives have an assigned spatial reference of geographic coordinates reduced to the World Geodetic System 84 datum reference for altitude (or depth) data (WGS84). Each of the physical and/or chemical parameters (Oxygen, Salinity and Temperature) has been set a “transparency” of 50% so that the raster grids