Left: Piston Coring Device. Middle: Split sediment core. Right: A Munsell color chart.
The ocean floor is carpeted with sediment that ranges from less than a meter in thickness on ocean crust recently created at the mid-ocean ridge spreading centers to more tham 10 km in thickness beneath continental shelves at the edges of the oceans. Since the late 1950s the Lamont-Doherty Earth Observatory of Columbia University has been recovering cores of the upper 5 to 20 meters of the sediment that has most recently settled on the seabed during the past few thousand to few million years. The cores are obtained as cylinders of sediment removed from a long pipe inserted into the seabed. The cylinders are split lengthwise in our core repository to reveal a cross-section of the sediment layers.
After photographing the exposed surface, the color, texture, mineral and micro-fossil content of all of the different sediment layers are recorded and entered into a database. There are currently more than 12,000 cores from all of the global ocean in the Observatory's Core Repository under the support of the U. S. National Science Foundation.
The sediment color of each layer is assigned its own code using Munsell Soil Color Charts. To create the Ocean Floor True Color Map, we extracted the color code for the topmost sediment layer and converted the color to a red-green-blue triplet for display on computer screens. The dots on the map have been given that color.
Left: Piston Coring Device. Middle: Split sediment core. Right: A Munsell color chart.
With our Earth Observer App we are introducing to the public the first global scene showing the true color of the seabed. This is a work in progress. Research is underway to relate the color to sediment province, distance from land, seabed depth, ocean bottom oxygen levels and surface ocean productivity in order to interpolate between the individal core locations and produce a seamless map.
Where the seabed has been investigated on a regular grid as along the US NortheastCoast margin of the North Atlantic Ocean, we can more readily recognize distinct regions of different color.
True sediment color on the US continental margin off New Jersey, New York and New England
The light tan colors immediately south of Long Island and on Georges Bank indicate coarse (mud-free) sediment rich in shells and mixed with sand and gravel delivered by melting ice from the last ice-age. The gray mud in Chesapeake Bay and Long Island Sound reflects seasonal low oxygen levels in the bottom waters. The medium brown color on the continental slope is from pure mud without sand or shells. Generally the darker the sediment, the finer the grainsize and the lighter the sediment, the coraser in grain size.
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