Stable isotope record of Cibicidoides spp. from Late Miocene sediments of the Southern Ocean

Deepwater circulation plays an important role in climate modulation through its redistribution of heat and salt and its control of atmospheric CO2. Oppo and Fairbanks (1987, doi:10.1016/0012-821X(87)90183-X) showed that the Southern Ocean is an excellent monitor of deepwater circulation changes for two reasons: (1) the Southern Ocean is a mixing reservoir for incoming North Atlantic Deep Water and recirculated water from the Pacific and Indian oceans; and (2) the nutrient/delta13C tracers of deepwater are not significantly changed by surficial processes within the Southern Ocean. We can extend these principles to the late Miocene because tectonic changes in the Oligocene and early and middle Miocene developed near-modern basinal configurations. However, on these time scales, changes in the oceanic carbon reservoir and mean ocean nutrient levels also affect the delta13C differences between ocean basins. From 9.8 to 9.3 Ma, Southern Ocean delta13C values oscillated between high North Atlantic values and low Pacific values. The Southern Ocean recorded delta13C values similar to Pacific values from 9.2 to 8.9 Ma, reflecting a low contribution of Northern Component Water (NCW). The delta13C differences between the NCW and Pacific Outflow Water (POW) end-members were low from 8.9 to 8.0 Ma, making it difficult to discern circulation patterns. NCW production may have completely shutdown at 8.6 Ma, allowing Southern Component Water (SCW) to fill the North Atlantic and causing the delta13C values in the North Atlantic, Pacific, and Southern oceans to converge. Deepwater delta13C patterns resembling the modern distributions evolved by 7.0 Ma: delta13C values were near 1.0 per mil in the North Atlantic; 0.0 per mil in the Pacific; and 0.5 per mil in the Southern Ocean. Development of near-modern delta13C distributions by 7.0 Ma resulted not only from an increase in NCW flux but also from an increase in deepwater nutrient levels. Both of these processes increased the delta13C difference between the North Atlantic and Pacific oceans. Deepwater circulation patterns similar to today's operated as early as 9.8 Ma, but were masked by the lower nutrient/delta13C differences. During the late Miocene, 'interglacial' intervals prevailed during intervals of NCW production, while 'glacial' intervals occurred during low NCW production.

Stable isotope record of Cibicidoides spp. from early and middle Miocene sediments

The middle Miocene delta18O increase represents a fundamental change in the ocean-atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ~20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high delta13C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep-waters warmed in the early Miocene by ~3°C (?20 to 16 Ma) and cooled by a similar amount during the middle Miocene delta18O increase (14.8 to 12.6 Ma), corresponding to the increase (?20 Ma) and subsequent decrease (~16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 per mil), relatively rapid (~0.5 m.y.) delta18O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long-term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene delta18O shift.

Geochemistry of minerals at DSDP Hole 70-504B

Hole 504B, drilled into the 5.9 Ma crust of the southern flank of the Costa Rica Rift, tapped a hydrothermal system in its conductive stage. Three alteration zones were encountered along the 561.5 meters of basement drilled. The upper alteration zone, 274.5 to 584.5 meters below the seafloor (BSF), is characterized by the presence of color zonation in which red halos are located between dark gray inner rock portions and dark gray outer bands. The red halos are characterized by an abundance of iddingsite, and they have higher K2O contents and Fe3+/FeT ratios, but lower SiO2 contents, than the adjacent dark gray inner zones. The dark gray outer bands are characterized by the presence of celadonite-nontronite. Saponite is omnipresent in these three alteration bands. Phillipsite is the only zeolite that occurs in the upper alteration zone. The upper alteration zone is interpreted as being the result of low-temperature alteration, with large amounts of cold oxygenated seawater percolating through the upper ocean crust. In the upper alteration zone, the formation of red halos was both preceded and followed by formation of dark gray outer bands. Then followed formation of dark gray cores. The lower alteration zone (584.5-835.5 m BSF) is characterized by the absence of color zonation, the downward-increasing abundance of pyrite and saponite, and the presence of quartz, talc, and calcite. The chemical changes (downhole MgO enrichment and concomitant CaO depletion) observed in the basalts of the lower alteration zone are thought to result from reactions of oceanic basalts with evolved seawater (i.e., solutions derived from seawater that has already reacted with ocean crust), which is thus depleted in oxygen, potassium, and radiogenic strontium. This alteration process, which was responsible for saponite formation in both the upper and lower alteration zones, was rock dominated, and it took place under suboxic to anoxic conditions during a second stage of alteration. Reaction temperatures could have progressively increased with depth. There is also a zeolitic zone that essentially coincides with the lower part of the upper alteration zone (between 528.5 and 563 m BSF). The host rock adjacent to veins of zeolite exhibits a greenish discoloration due to the intensive replacement of the igneous minerals. The replacement minerals result in significant increases in the bulk rock K2O, MgO, CaO, CO2, and H2O+ contents. The solutions circulating along the newly opened fissures had high Ca activity, and minerals probably precipitated in these fissures at 60°C or 110°C. These hydrothermal solutions circulated later than those responsible for the formation of the minerals that characterize the upper and lower alteration zones.