Woods Hole Oceanographic Institution, Woods Hole, MA, United States

Grayscale analysis on standard black and white core photographs from Ocean Drilling Program (ODP) Leg 162 (Arctic-Atlantic Gateways II) was employed to determine calcium carbonate concentrations. Carbonate concentrations determined by this optical method were tested against laboratory measured values. We introduce a new grayscale analysis methodology for eliminating light distortions. Earlier methods did not differentiate between intraphotograph and interphotograph lighting distortions; additionally, these previous methods incorrectly assumed that these distortions affected all colors equally. In reality, dark and light colors are relatively insensitive to light distortions, while midtones are very responsive to lighting variations. In addition, as the overall color of a core photograph darkens, the color with the greatest response to distortions lightens. The methodology utilized in this work accounts for all of these factors. Equations correlating carbonate content with grayscale value were also developed. There is a strong linear correspondence between calcium carbonate concentration and sediment color, as measured by grayscale values. These relationships allow us to predict calcium carbonate variations with a Root-Mean-Square error of + or -9.67 wt.% over the carbonate range (from 3.5 wt.% to 78.7 wt.%). Several single variable grayscale equations predict carbonate variations almost identically. In general, the grayscale equations tend to somewhat overestimate calcium carbonate at very low concentrations (<15 wt. %) and underestimate it at higher concentrations (>70 wt.%). However in all cases, the improved analysis method produces better results than existing calibration methods. Grayscale analysis was also performed on core photographs from ODP Legs 138 (North Pacific) and 154 (equatorial Atlantic) with similar results, suggesting the potential for wide-spread applicability of this methodology.