Preservation and color differences in Nuttallides umbonifera

摘要

Abundant Nuttallides umbonifera (Cushman) in Holocene deep sea sediments of the western North Atlantic make them of great value for reconstructing deep sea paleo-environments. This species is particularly abundant in the high-deposition-rate sediments of the Laurentian Fan between Nova Scotia and Newfoundland, where variations in test color came to our attention. Specimens ranged from a white, chalky appearance to a golden, translucent brown. Because we have conducted stable isotope studies using this species, we reasoned it is important to know the basis for the different colors. Living N. umbonifera are brown due to an internal, pigmented, chitinous layer of the test. In the natural environment, tests could be etched by corrosive bottom waters, or by corrosive pore waters on the millimeter to centimeter scale. By lightly subjecting brown specimens to dilute acid, we are able to etch and frost them until their tests appeared to lose their brown color and turn white. By measuring delta(13)C and delta(18)O on large individuals from the same sample, we show that, on average, there is no significant isotopic difference between the brown and white specimens. Thus, we rule out changing bottom water properties as the source of dissolution. Instead, it is likely that bioturbation creates local pockets of reducing conditions which cause etching of some (but not all) specimens in a contemporaneous population.