Stable isotope ratios of epibenthic foraminifer Cibicidoides wuellerstorfi from sediment profile GeoB9526

Studies of temporal changes of ocean circulation and deep-water ventilation often rely on d13C records of epibenthic foraminifer Cibicidoides wuellerstorfi. However, primary productivity related overprints may distort the signal and simulate a chemical age of ambient water mass that is too old and simulates poorly ventilated ambient bottom waters. To further constrain the use of C. wuellerstorfi d13C records from high-productivity areas, we analyzed a 14CAMS-dated gravity core from the upwelling regime off northwest Africa at 12°N. We compare this new record with 37 radiocarbon dated d13C records from the eastern Atlantic Ocean between 45°N and 25°S that are bathed by the same water mass. Only during Heinrich events 1 and 2, when the investigated core site off northwest Africa experienced year-round, sustained deposition of organic matter, the d13C values at this site faithfully record deep-water ventilation states. During times of predominantly seasonal deposition of fresh phytodetritus, however, d13C values were significantly lower than at the reference sites. This underscores that reconstruction of paleocirculation and deep ocean ventilation using C. wuellerstorfi d13C from regions that experienced seasonal phytodetritus deposition needs to be validated by additional proxies that are not affected by local productivity.

(Table 1) Cd/Ca and Mn/Ca ratios of benthic foraminifera from DSDP Hole 30-289

The late Miocene carbon shift (~6.2 Myr) -a 0.5-1.0 per mil, d13C decrease in benthic and planktonic foraminifera- has been ascribed to changes in global inventory, deep-ocean circulation, and/or productivity. Cadmium, d13C, and nutrients in the ocean are linked; comparison of d13C and Cd/Ca yields circulation and chemical inventory information not available from either alone. We determined Cd/Ca ratios in late Miocene benthic foraminifera from DSDP Site 289. Results include: (1) late Miocene Pacific Cd/Ca values fall between those of late Quaternary Atlantic and Pacific benthic foraminifera; (2) there are no systematic Cd/Ca offsets between Cibicidoides kullenbergi, Cibicidoides wuellerstorfi and Uvigerina spp.; and (3) there is a very slight Cd/Ca change coincident with d13C. Cd/Ca, slightly higher in younger, isotopically lighter samples, exhibits a smaller increase than predicted if circulation were the primary cause of the carbon shift. The carbon shift may have been due to a long-term shift in the steady-state carbon isotope input or to a change in the sedimentation of organic carbon relative to calcium carbonate.

Compilation of 220 sediment core d13C data from the glacial Atlantic Ocean

We compare a compilation of 220 sediment core d13C data from the glacial Atlantic Ocean with three-dimensional ocean circulation simulations including a marine carbon cycle model. The carbon cycle model employs circulation fields which were derived from previous climate simulations. All sediment data have been thoroughly quality controlled, focusing on epibenthic foraminiferal species (such as Cibicidoides wuellerstorfi or Planulina ariminensis) to improve the comparability of model and sediment core carbon isotopes. The model captures the general d13C pattern indicated by present-day water column data and Late Holocene sediment cores but underestimates intermediate and deep water values in the South Atlantic. The best agreement with glacial reconstructions is obtained for a model scenario with an altered freshwater balance in the Southern Ocean that mimics enhanced northward sea ice export and melting away from the zone of sea ice production. This results in a shoaled and weakened North Atlantic Deep Water flow and intensified Antarctic Bottom Water export, hence confirming previous reconstructions from paleoproxy records. Moreover, the modeled abyssal ocean is very cold and very saline, which is in line with other proxy data evidence.

Epibenthic foraminiferal d13C in the Recent deep Arctic Ocean

Low planktic and benthic d18O and d13C values in sediments from the Nordic seas of cold stadials of the last glaciation have been attributed to brines, formed similar to modern ones in the Arctic Ocean. To expand on the carbon isotopes of this hypothesis I investigated benthic d13C from the modern Arctic Ocean. I show that mean d13C values of live epibenthic foraminifera from the deep Arctic basins are higher than mean d13C values of upper slope epibenthic foraminifera. This agrees with mean high d13C values of dissolved inorganic carbon (DIC) in Arctic Bottom Water (ABW), which are higher than mean d13CDIC values from shallower water masses of mainly Atlantic origin. However, adjustments for oceanic 13C-Suess depletion raise subsurface and intermediate water d13CDIC values over ABW d13CDIC ones. Accordingly, during preindustrial Holocene times, the d13CDIC of ABW was as high or higher than today, but lower than the d13CDIC of younger subsurface and intermediate water. If brine-enriched water significantly ventilated ABW, brines should have had high d13CDIC values. Analogously, high-d13CDIC brines may have been formed in the Nordic seas during warm interstadials. During cold stadials, when most of the Arctic Ocean was perennially sea-ice covered, a cessation of high-d13CDIC brine rejection may have lowered d13CDIC values of ABW, and ultimately the d13CDIC in Nordic seas intermediate and deep water. So, in contrast to the idea of enhanced brine formation during cold stadials, the results of this investigation imply that a cessation of brine rejection would be more likely.

Benthic foraminifera assemblages of the Gulf of Aden

Benthic and selected planktic foraminifera and stable isotope records were determined in a piston core from the Gulf of Aden, NW Arabian Sea that spans the last 530 ka. The benthic foraminifera were grouped into four principal assemblages using Q-mode Principal Component Analyses. Comparison of each of these assemblages with the fauna of the nearby regions enabled us to identify their specific environmental requirements as a function of variability in food supply and strength of the oxygen minimum zone and by that to use them as indicators of surface water productivity. The benthic foraminiferal productivity indicators coupled with the record of Globigerina bulloides, a planktic foraminifer known to be sensitive to productivity changes in the region, all indicate higher productivity during glacial intervals and productivity similar to present or even reduced during interglacial stages. This trend is opposite to the productivity pattern related to the SW summer monsoon of the Arabian Sea and indicates the role of the NE winter monsoon on the productivity of the Gulf of Aden. A period of exceptionally enhanced productivity is recognized in the Gulf of Aden region between ~60 and 13 kyr indicating the intensification of the NE winter monsoon to its maximal activity. Contemporaneous indication of increased productivity in other parts of the Arabian Sea, unexplained so far by the SW summer monsoon variability, might be related to the intensification of the NE winter monsoon. Another prominent event of high productivity, second in its extent to the last glacial productivity event is recognized between 430 and 460 kyr. These two events seem to correspond to periods of similar orbital positioning of rather low precession (and eccentricity) amplitude for a relatively long period. Glacial boundary conditions seem to control to a large extent the NE winter monsoon variability as also indicated by the dominance of the 100 ka cycle in the investigated time series. Secondary in their importance are the 23 and 41 ka cycles which seem also to contribute to the NE monsoonal variability. Following the identification of productivity events related to the NE winter monsoon in the Gulf of Aden, it is possible now to extend this observation to other parts of the Arabian Sea and consider the contribution of this monsoonal system to the productivity fluctuations preserved in the sedimentary records.

Ground truthing the Cd/Ca-carbon isotope relationship in foraminifera of the Greenland-Iceland-Norwegian Seas

In order to examine whether the paleoceanographic nutrient proxies, d13C and cadmium/calcium in foraminiferal calcite, are well coupled to nutrients in the region of North Atlantic Deep Water formation, we present da ta from two transects of the Greenland-Iceland-Norwegian Seas. Along Transect A (74.3°N, 18.3°E to 75.0°N, 12.5°W, 15 stations), we measured phosphate and Cd concentrations of modern surface sea water. Along Transect B (64.5°N, 0.7°W to 70.4°N, 18.2°W, 14 stations) we measured Cd/Ca ratios and d13C of the planktonic foraminifera Neogloboquadrina pachyderma sinistral in core top sediments. Our results indicate that Cd and phosphate both vary with surface water mass and are well correlated along Transect A. Our planktonic foraminiferal d13C data indicate similar nutrient variation with water mass along Transect B. Our Cd/Ca data hint at the same type of nutrient variability, but interpretations are hampered by low values close to the detection limit of this technique and therefore relatively large error bars. We also measured Cd and phosphate concentrations in water depth profiles at three sites along Transect A and the d13C of the benthic foraminifera Cibicidoides wuellerstorfi along Transect B. Modern sea water depth profiles along Transect A have nutrient depletions at the surface and then constant values at depths greater than 100 meters. The d13C of planktonic and benthic foraminifera from Transect B plotted versus depth also reflect surface nutrient depletion and deep nutrient enrichment as seen at Transect A, with a small difference between intermediate and deep waters. Overall we see no evidence for decoupling of Cd/Ca ratio and d13C in foraminiferal calcite from water column nutrient concentrations along these transects in a region of North Atlantic Deep Water formation.

Stable isotope analysis and temperature reconstruction data from DSDP Hole 94-609B and ODP Hole 162-984B

The Greenland ice sheet is accepted as a key factor controlling the Quaternary glacial scenario. However, the origin and mechanisms of major Arctic glaciation starting at 3.15 Ma and culminating at 2.74 Ma are still controversial. For this phase of intense cooling Ravelo et al. proposed a complex gradual forcing mechanism. In contrast, our new submillennial-scale paleoceanographic records from the Pliocene North Atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation (NHG), since it was linked to a 2-3 °C surface water warming during warm stages from 2.95 to 2.82 Ma. These records support previous models, claiming that the final closure of the Panama Isthmus (3.0- ~2.5 Ma induced an increased poleward salt and heat transport. Associated strengthening of North Atlantic Thermohaline Circulation and in turn, an intensified moisture supply to northern high latitudes resulted in the build-up of NHG, finally culminating in the great, irreversible climate crash at marine isotope stage G6 (2.74 Ma). In summary, there was a two-step threshold mechanism that marked the onset of NHG with glacial-to-interglacial cycles quasi-persistent until today.

Dinoflagellate cyst counts and age model from sediment samples of ODP Hole 175-1085A

Sediment samples from ODP Site 1085 were investigated in order to obtain more information on the initiation and development of the Benguela upwelling system during the middle and upper Miocene. In particular, our intent was to establish the causes of the upwelling as well as the response of the upwelling regime to the development of the Antarctic Circumpolar Current. Based on changes in the calcareous dinoflagellate cyst association, we found an initial increase of the dinoflagellate cyst productivity, probably related to the initiation of upwelling about 11.8 Ma ago. Two distinct increases in cyst productivity in conjunction with temperature decreases of the upper water masses reflect upwelling pulses off Namibia and occur at the end of the Miocene cooling events Mi5 (about 11.5 Ma) and Mi6 (about 10.5 Ma). Both cooling events are associated with an ice volume increase in Antarctica and are thought to have led to an increase in southeasterly winds, possibly causing these two upwelling pulses. We demonstrate a decrease in dinoflagellate cyst productivity and enhanced terrigenous input via the Orange River after the Mi5 event. At about 11.1 Ma, the dinoflagellate cyst productivity increases again. The polar cyst species Caracomia arctica occurs here for the first time. This implies an influence of subantarctic mode water and therefore a change in the quality of the upwelling water which allowed the Benguela upwelling to develop into modern conditions. From about 10.4 Ma, C. arctica forms a permanent part of the association, pointing to an establishment of the upwelling regime.

Paleoceanographic data from cores M23414 and PS1243

Variations in the poleward-directed Atlantic heat transfer was investigated over the past 135 ka with special emphasis on the last and present interglacial climate development (Eemian and Holocene). Both interglacials exhibited very similar climatic oscillations during each preceding glacial terminations (deglacial TI and TII). Like TI, also TII has pronounced cold-warm-cold changes akin to events such as H1, Bølling/Allerød, and the Younger Dryas. But unlike TI, the cold events in TII were associated with intermittent southerly invasions of an Atlantic faunal component which underscores quite a different water mass evolution in the Nordic Seas. Within the Eemian interglaciation proper, peak warming intervals were antiphased between the Nordic Seas and North Atlantic. Moreover, inferred temperatures for the Nordic Seas were generally colder in the Eemian than in the Holocene, and vice versa for the North Atlantic. A reduced intensity of Atlantic Ocean heat transfer to the Arctic therefore characterized the Eemian, requiring a reassessment of the actual role of the ocean-atmosphere system behind interglacial, but also, glacial climate changes.