Spatial distribution of isotopic compositions of Globigerina bulloides and Neogloboquadrina pachyderma (dex.) and the faunal assemblages of planktic foraminifera in surface sediment samples along the Chilean continental slope between 23°S and 44°S

We report on the spatial distribution of isotopic compositions of the two planktic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.), and the faunal assemblages of planktic foraminifera in 91 surface sediment samples along the Chilean continental slope between 23°S and 44°S. Both d13C and d18O data of N. pachyderma (dex.) show little variability in the study area. North of 39°S, the isotopic values of N. pachyderma (dex.) are heavier than those of G. bulloides, whereas south of 39°S, this relation inverses. This is indicative for a change from a well-mixed, deep thermocline caused by coastal upwelling north of 39°S to well-stratified water masses in a non-upwelling environment south of 39°S. In addition, the faunal composition of planktic foraminifera marks this change by transition from an upwelling assemblage north of 39°S to a high-nutrient-non-upwelling assemblage south of 39°S, which is characterized by decreased contributions of upwelling indicators such as G. bulloides, N. pachyderma (sin.), and Globigerinita glutinata. In general, we can conclude that food and light rather than temperature are the primary control of the planktic foraminiferal assemblage between 23°S and 44°S off Chile. Our data point to higher marine productivity at upwelling centers north of 25°S and at 30-33°S. South of 39°S, significant supply of nutrients by fluvial input most likely boosts the productivity.

(Table 1) Trace element and isotopic data for ODP Hole 145-887D basalts

Age-progressive, linear seamount chains in the northeast Pacific appear to have formed as the Pacific plate passed over a set of stationary hotspots; however, some anomalously young ages and the lack of an "enriched" isotopic signature in basalts from the seamounts do not fit the standard hotspot model. For example, published ages (28-30 Ma) for basalts dredged from the Patton-Murray seamount platform in the Gulf of Alaska are 2-4 m.y. younger than the time when the platform was above the Cobb hotspot. However, the lowermost basalt recovered by ocean drilling on Patton-Murray yielded a 40Ar-39Ar age of 33 Ma. This age exactly coincides with the time when the seamount platform was above the Cobb hotspot, consistent with a stationary, long-lived mantle plume. A 27 Ma alkalic basalt flow recovered 8 m above the 33 Ma basalt is similar in age and composition to the previously dredged basalts, and may be the alkalic capping phase typical of many hotspot volcanoes. A 17 Ma tholeiitic basalt sill recovered 5 m above the 27 Ma basalt was emplaced long after the seamount platform moved away from the hotspot, and may be associated with a period of intraplate extension. Anomalously young phases of volcanism on this and other hotspot seamounts suggest that they can be volcanically rejuvenated by nonhotspot causes, but this rejuvenation does not rule out the hotspot model as an explanation for the initial creation of the seamount platform. The lack of an "enriched" isotopic signature in any of these basalts shows that enriched compositions are not necessary characteristics of plume-related basalts. The isotopic compositions of the lower basalts are slightly more depleted than the 0-9 Ma products of the Cobb hotspot, despite the fact that the hotspot was closer to a spreading ridge at 0-9 Ma. It appears that this hotspot, like several others, has become more enriched with time.