Changes in upper water-column structure at Site 925, late Miocene–Pleistocene: planktonic foraminifer assemblage and isotopic evidence

Abstract

The relative abundance of all planktonic foraminifer species and the d 18 O and d 13 C values of three species ( Globigerinoides sacculifer, a mixed-layer dweller; Neogloboquadrina dutertrei, a thermocline dweller; and Truncorotalia crassaformis, a deep dweller) was determined in 25 samples at ~250 k.y. intervals through the last 6 m.y. at Ocean Drilling Program Site 925. Combined, the data indicate that mixed-layer depth was a minimum at the end of the early Pliocene (4.0 Ma) and gradually increased toward the Pleistocene.The d 18 O and d 13 C gradients in the mixed layer and the thermocline were derived by subtracting isotopic values of Globigerinoides sacculifer from Neogloboquadrina dutertrei and values of N. dutertrei from Truncorotalia crassaformis. Changes in d 18 O gradients were used as a proxy record for variation in thermocline strength and position, while changes in d 13 C gradients served a like purpose for productivity variation.The early Pliocene was characterized by relatively high numbers of thermocline-dwelling taxa, relatively small mixed-layer d 18 O gradients, and relatively large d 13 C gradients. These indicators suggest that downwelling was at a minimum for the last 6 m.y. during this period at Site 925, the temperature difference between G. sacculifer and N. dutertrei habitats was small, and surface productivity was relatively great. After 2.5 Ma thermocline-dwelling speciesÕ numbers were relatively low, mixed-layer d 18 O gradients were relatively large, and mixed-layer d 13 C were relatively small. The increased pole-to-equator temperature gradient that accompanied the growth of Northern Hemisphere ice sheets caused the intertropical convergence zone to move toward the equator and exert a growing inßuence at Site 925 through the last 4 m.y.

Warm ocean processes and carbon cycling in the Eocene

“…(iv) The seasonal upwelling effect. Certain species, including Globigerina bulloides and the small species Globigerinita glutinata, as discussed in the study of with Neogloboquadrina dutertrei [74][75][76], are indicative of more productive tropical surface conditions linked to seasonal upwelling [75][76][77]. The δ 13 C value of such species' tests is therefore more similar to that of deeper-dwelling species because they record the isotopic chemistry of cool, 12 C-rich, upwelling deepwaters.…”

Pliocene Changes in Tropical East Pacific Upper Ocean Stratification: Response to Tropical Gateways?

“…Moreover, support for an early Pliocene southward shift of the ITCZ comes from studies that investigated Atlantic and Pacific changes in tropical wind field and surface hydrography (Hovan, 1995;Farrell et al, 1995;Cannariato and Ravelo, 1997;Chaisson and Ravelo, 1997;Norris, 1998;Billups et al, 1998Billups et al, , 1999. Similar to the modern seasonal cycle, a shift to a more southern position of the ITCZ in the TEP would have weakened the southeast trades and thus significantly reduced the strength of the NECC and the SEC.…”

Insight into the latest Messinian (5.7–5.2Ma) palaeoclimatic events from two deep-sea Atlantic Ocean ODP Sites

“…Indeed, increased south-easterly trade winds result in piling up of warm surface waters in the western tropical Atlantic (Chaisson and Ravelo, 1997). Therefore, the presence of high percentages of mixed layer species (in particular before 5.6 Ma and during the glacial following TG11) supports the hypothesis that the late Messinian glaciations were accompanied by enhanced SE trade winds.…”

“…In the case of the mixed layers species ( Figure 2I) individual species were grouped following known ecological preferences and according to Chaisson and Ravelo (1997). For ODP Site 925, the preservation of calcium carbonate (CaCO 3 ) was also assessed using the percentages of planktonic foraminifera fragments from Pfuhl and Shackleton (2004) as well as the individual shell weights obtained prior to the trace metal analysis.…”

“…Other faunal results, expressed as a percentage of the total for selected indicative species or groups of species such as mixed layer species Chaisson and Ravelo (1997), % "Menardellidae" and % Neogloboquadrina pachyderma (left and right coiling) ( Figure 2I, J, K respectively).…”