MIOCENE TO RECENT CHANGES IN OCEAN FLOOR SEDIMENT CORE CHEMISTRY AND MINERALOGY FROM THE NORTH ATLANTIC AND GULF OF MEXICO RECORD THE OPENING OF THE ATLANTIC OCEAN

The first post-WWII program of systematic global mapping and sampling of the world's oceans and the seafloor beneath were conducted by two ships, the R.V. Vema and R.V. Robert D. Conrad, both of which operated out of Lamont-Doherty Earth Observatory at Columbia University. Chemical, mineralogical and microscopic study of thirteen North Atlantic and Gulf of Mexico cores from those early voyages reveals systematic changes from Miocene to recent sediments that reflect the opening of the Atlantic and the development of passive margin sedimentation. XRD analysis reveals significant differences between the clay mineral composition of the Atlantic and Gulf samples related to the origin of sediments. The Atlantic samples demonstrate a high amount of Chlorite and Illite from the weathering of the Appalachians. The Gulf of Mexico samples are smectite-rich due to erosion of Cretaceous and Tertiary sediments from the western interior, plus detrital kaolinite, illite and some chlorite. These data are consistent with earlier work by Pinsak & Murray (7th Nat’l Conf. Clays & Clay Min., 162-177, 1958). The deeper portions of some Atlantic cores contain smectite, suggesting some influence from early stages of volcanic activity along the midocean ridge. Otherwise, little vertical variation is seen. A small amount of gypsum occurs in some surface samples. SEM photos of selected samples show the foraminifera Neogloboquadrina dutertrei (?) and poorly preserved specimens of the coccolith Homozygosphaera triarcha (?). Trace element discrimination diagrams were constructed to compare compositional variation with data from known tectonic settings. Following the methods of Ryan & Williams (Chem. Geol. 242, 103-125, 2007) and Bhatia & Crook (Contrib. Min. Pet. 92, 181-193, 1986) we find that Rb, Sr, Nb, Y, Zr, Ti, Th and Cr are particularly effective discriminators. North Atlantic cores located near the mid-Atlantic ridge and away from the continental margin show a progressive decrease in Rb/Sr, Y, Th and Zr/TiO2 from bottom to top, reflecting a decrease through time in the effects of an active spreading ridge. Samples collected close to the continental margin in both the Atlantic and the Gulf of Mexico exhibit a reverse trend, as progressive erosion of Appalachian basement rocks becomes a dominant influence on detrital sediment composition.