Alkenone analyses of sediment cores from the western Arabian Sea

Records of total organic carbon (TOC) and C37 alkenones were used as indicators for past primary productivity in the western and eastern Arabian Sea. Data from GeoB 3005, an open ocean site in the western Arabian Sea upwelling area, are compared with similar records of GeoB 3007 from the Owen Ridge, Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the eastern Arabian Sea. TOC/C37 alkenone records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing all over the Arabian Sea. Based on their phase-relationship to variations in boreal summer insolation they can be divided into three groups. In the western Arabian Sea the precession-related phasing is different between productivity proxies and those for summer monsoon wind strength and upwelling intensity. TOC and C37 alkenone records from the western Arabian Sea lag the other monsoonal indicators by about 5 kyr, but lead productivity indicators from the eastern Arabian Sea by 3 kyr. Based on the differences in phase relationships associated with the precessional cycling between productivity and monsoonal proxies in the western Arabian Sea it is proposed that the TOC/C37 alkenone signal in the western Arabian Sea document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds. In the eastern Arabian Sea the phasing hints to coincidence between maximum productivity and stronger NE monsoon winds associated with precession-related maxima in ice volume. In contrast, variations in paleoproductivity at site GeoB 3007 from the Owen Ridge indicate productivity maxima during glacial substages 8.2, 6.2 and 2.2, whereas precessionrelated changes are of only minor importance at this location. The results of frequency analyses confirm that productivity at site GeoB 3007 responds predominantly to glacialinterglacial climate changes, while site GeoB 3005 from the open ocean upwelling region near the Gulf of Aden is dominated by precessional insolation. A possible explanation for the pattern revealed at the Owen Ridge is the periodic NW-SE displacement of the Findlater Jet axis, which separates the region of open ocean upwelling to the northwest from downwelling to the southeast ofthe jet. The carbon isotopes of planktic foraminifera reflect nutrient related d13C variations of dissolved inorganic carbon. The difference between the planktic foraminifera Globigerinoides ruber (w), living in the upper 50 m of the water column, and the deeper Iiving Neogloboquadrina dutertrei (Delta d13Cr-d) of core GeoB 3005 displays nutrient variations in the upwelling area near the Gulf of Aden. The results of cross-spectral analyses between Deltad13Cr-d of GeoB 3005 and proxies for SW monsoon intensity indicate, too, a dissociation of productivity from monsoonal upwelling intensity. Instead, productivity depends mainly on the availability of nutrients, while upwelling intensity of sub-surface water masses seems to be of only secondary importance. Additionally, sea surface temperatures (SSTs) were reconstructed using the unsaturation ratio of C37 alkenones. Alkenone SSTs reflect annual mean temperatures rather than explicitly the season of upwelling. This is evident from alkenone SSTs in a transect of surface sediments extending from the inner Gulf of Aden into the western Arabian Sea. The alkenone-derived SST records of GeoB 3005 from the open ocean upwelling region near the Gulf of Aden and GeoB 3007 from the Owen Ridge reveal similar variations with high SSTs during interglacial and low SSTs during glacial periods. The glacial marine oxygen isotope stage (MIS) 6 remains relatively warm and was not as cold as MIS 3 to 4 and 8 according to the alkenone SST. Similar variation-patterns were reconstructed in the coastal upwelling area off Oman for ODP Site 723 as weIl as in the eastern Arabian Sea for MD 900963, where upwelling is not as pronounced as in the western Arabian Sea. Spectral-analyses indicate that SST changes are in good agreement with the modulation of low-latitude precessional insolation changes by eccentricity. However, they do not show the pronounced cydicity in the precessional frequency band, which is characteristic for variations in paleoproductivity. Although the overall variation pattern is very similar, a dose comparison between the western (GeoB 3005) and the eastern Arabian Sea (MD 900963) shows larger differences between both sites during cold intervals than during periods of warm SSTs. This is attributed to a more effective cooling of surface waters in the western Arabian Sea by prolonged NE monsoon winds during times of expanded Northern Hemisphere ice-sheets, thereby lowering the annual mean SSTs stronger than in the eastern Arabian Sea.

(Table 2) Analyses of prominent ash layers recovered at DSDP Site 18-178, Gulf of Alaska

A sequence of ash layers recovered from site 178 of the Deep Sea Drilling Project in the Gulf of Alaska was studied to determine the nature of highly explosive volcanic eruptions associated with the Aleutian Arc and Alaskan Peninsula during the last 8 m.y. The major-element chemistry of 25 distinct ash layers was determined. When the analyses are plotted on conventional major-element variation diagrams, the unusual, highly evolved, calc-alkalic characteristics of the ashes are apparent. Perhaps more significantly, there is a good correlation of certain indices of the degree of chemical evolution of each ash (SiO2 content and Larsen index) with sample age. Both parameters vary cyclically, with maximum values of both indices occurring at present, 2.5, and about 5.0 m.y. ago. The cause of the cyclic activity, as well as discontinuous volcanic activity reported for other areas by other investigators, is not precisely known. However, we suggest that variable rates of subduction provide a viable hypothesis for discontinuous volcanic activity associated with convergent plate boundaries.