Orbitally-paced Carbonate Dissolution During the Paleocene

Early Paleogene deep-sea sediments that contain orbital-scale lithologic cycles (relative changes in carbonate content) have been recovered from the northwest Atlantic Ocean (ODP Leg 171B), the southwest Atlantic Ocean (DSDP Leg 84 and ODP Leg 208), the southeast Atlantic Ocean (DSDP Legs 39 and 72), the Southern Ocean (ODP Leg 113), and the northwest Pacific (ODP Leg 198). The cyclic lithology has been correlated among all locations with sufficiently long and continuous records, implying a global-scale controlling mechanism - orbitally forced changes in insolation. We seek to understand how orbitally driven changes in insolation impacted climate during greenhouse intervals - the first step is to determine how the input signal is recorded in sediments. Several aspects of the climate system ultimately dictate carbonate content in deep-sea sediments making carbonate content a sensitive recorder of orbital cyclicity. A change in sedimentary carbonate content may result from a change in surface water production, dissolution, terrigenous sediment supply, or a combination of these factors. Each of these factors is, in turn, a function of prevailing climate parameters. Our work on the eolian fraction at Shatsky Rise indicates that the lithologic variations in the Pacific resulted from a combination of changes in terrigenous sediment supply and carbonate dissolution. Here we test the hypothesis that cyclic carbonate dissolution was caused by changes in deep-water aging patterns. We have analyzed the Nd isotopic composition of fossil fish debris to reconstruct the deep-water mass composition of the northern tropical Pacific at Shatsky Rise ODP Site 1209. The Nd isotope analyses span a set of nine consecutive short eccentricity cycles between 59 and 58 Ma, however we focused our analyses on the samples corresponding to eccentricity maxima and minima. Nd isotope values spanning the study interval vary between -2.9 to -4.0 epsilon units, however there is no systematic correlation between Nd isotope variability and sediment lithology. Thus water mass composition was not influenced by eccentricity forcing and changes in deep-water aging likely did not play a role in the cyclic dissolution evident at Shatsky Rise. The apparent lack of correlation between cyclic lithology and water mass composition implies that regional changes in the depth of the lysocline caused the cyclic increase in carbonate dissolution. Because the cyclic lithologic changes are globally correlative, the inferred changes in lysocline depth were likely global-scale phenomena. Nd isotopic reconstructions of water mass chemistry spanning the same cycles at Walvis Ridge ODP Site 1262 also indicate no relationship between cyclic dissolution and water mass composition, leaving changes in lysocline depth as the cause of dissolution. Concurrent lysocline shifts in the Atlantic and Pacific support the global nature of the changes, and the most likely cause of global-scale changes in lysocline depth during a greenhouse climate interval is changes in the concentration of atmospherics CO2.