Stable oxygen isotope. Mg/Ca and Temperature record of Caribbean and Pacific sediments

The final phase of the closure of the Panamanian Gateway and the intensification of Northern Hemisphere Glaciation (NHG) both occurred during the Late Pliocene. Glacial-interglacial (G-IG) variations in sea level might, therefore, have had a significant impact on the remaining connections between the East Pacific and the Caribbean. Here, we present combined foraminiferal Mg/Ca and d18O measurements from Ocean Drilling Program (ODP) Site 1241 from the East Pacific and ODP Site 999 from the Caribbean. The studied time interval covers the first three major G-IG Marine Isotope Stages (MIS 95-100, ~2.5 Ma) after the intensification of NHG. Analyses were performed on the planktonic foraminifera Neogloboquadrina dutertrei and Globigerinoides sacculifer, representing water mass properties in the thermocline and the mixed-layer, respectively. Changes in sea water temperature, relative salinity, and water column stratification strongly suggest that the Panamanian Gateway temporarily closed during glacial MIS 98 and 100, as a result of changes in ice volume equivalent to a drop in sea level of 60-90 m. Reconstructed sea surface temperatures (SST) from G. sacculifer show a glacial decrease of 2.5°C at Site 1241, but increases of up to 3°C at Site 999 during glacial MIS 98 and 100 suggesting that the Panamanian Gateway closed during these glacial periods. The Mg/Ca-temperatures of N. dutertrei remain relatively stable in the East Pacific, but do show a 3°C warming in the Caribbean at the onset of these glacial periods suggesting that the closing of the gateway also changed the water column stratification. We infer that the glacial closure of the gateway allowed the Western Atlantic Warm Pool to extend into the southern Caribbean, increasing SST (G. sacculifer) and deepening the thermocline (N. dutertrei). Additionally, ice volume appears to have become large enough during MIS 100 to survive the relatively short lasting interglacial MIS 99 so that the gateway remained closed. Towards the end of MIS 98, during MIS 97 and into MIS 96 temperatures on both sides are mostly similar suggesting water masses exchanged again. Additionally, Caribbean variations in SST and d18Owater follow a precession-like cyclicity rather than the obliquity-controlled variations characteristic of the East-Pacific and many other tropical areas, suggesting that regional atmospheric processes related to the trade winds and the Intertropical Convergence Zone (ITCZ) had a dominant impact in the Caribbean.

Crabeater seals (Lobodon carcinophaga) in the Weddell Sea during DRE1998 campaign

The crabeater seal (Lobodon carcinophaga) is the most abundant Antarctic seal and inhabits the circumpolar pack ice zone of the Southern Ocean. Until now, information on important environmental factors affecting its distribution as well as on foraging behaviour is limited. In austral summer 1998, 12 crabeater seals of both sexes and different age classes were equipped with satellitelinked dive recorders at Drescher Inlet (72.85°S, 19.26°E), eastern Weddell Sea. To identify suitable habitat conditions within the Weddell Sea, a maximum entropy (Maxent) modelling approach was implemented. The model revealed that the eastern and southern Weddell Sea is especially suitable for crabeater seals. Distance to the continental shelf break and sea ice concentration were the two most important parameters in modelling species distribution throughout the study period. Model predictions demonstrated that crabeater seals showed a dynamic response to their seasonally changing environment emphasized by the favoured sea ice conditions. Crabeater seals utilized ice-free waters substantially, which is potentially explained by the comparatively low sea ice cover of the Weddell Sea during summer 1998. Diving behaviour was characterized by short (>90 % = 0-4 min) and shallow (>90 % = 0-51 m) dives. This pattern reflects the typical summer and autumn foraging behaviour of crabeater seals. Both the distribution and foraging behaviour corresponded well with the life history of the Antarctic krill (Euphausia superba), the preferred prey of crabeater seals. In general, predicted suitable habitat conditions were congruent with probable habitats of krill, which emphasizes the strong dependence on their primary prey.

Uranium and thorium isotopes and sedimentation rates in metalliferous sediments from the western flank of the East Pacific Rise at 21°-22°S

Isotopic compositions of uranium (234U and 238U) and thorium (230Th and 232Th) were measured in metalliferous sediments from the western flank of the East Pacific Rise at 21°-22°S, in the area of hydrothermal activity and massive sulfide accumulation at the axis of the EPR. Concentration of 232Th (on the carbonate-free base) is consistent with composition of mafic extrusive rocks; isotope ratios 232Th/238U and 234U/238U indicate that about 70% of uranium passes into sediments from sea water with hydrothermal iron hydroxide. Mean sedimentation rates are calculated for seven cores by the nonequilibrium 230Th method with use of the constant concentration model. Flux of 230Th to bottom sediments is calculated and its mean value is used to determine sedimentation rate in four other cores. The constant flux model is used to calculate change of sedimentation rate with depth for seven cores over time interval of 100-300 ky. Sedimentation rates varied not much (0.3-0.6 cm/ky). The greatest changes occurred in two cores: one located near massive sulfide structures, and another near the spreading axis. Determinations of mean rates by the radiocarbon method and the nonequilibrium thorium method are in good agreement.

Middle Eocene to late Oligocene calcareous nannofossils of ODP Hole 120-748B

A major deterioration in global climate occurred through the Eocene-Oligocene time interval, characterized by long-term cooling in both terrestrial and marine environments. During this long-term cooling trend, however, recent studies have documented several short-lived warming and cooling phases. In order to further investigate high-latitude climate during these events, we developed a high-resolution calcareous nannofossil record from ODP Site 748 Hole B for the interval spanning the late middle Eocene to the late Oligocene (~42 to 26 Ma). The primary goals of this study were to construct a detailed biostratigraphic record and to use nannofossil assemblage variations to interpret short-term changes in surface-water temperature and nutrient conditions. The principal nannofossil assemblage variations are identified using a temperate-warm-water taxa index (Twwt), from which three warming and five cooling events are identified within the middle Eocene to the earliest Oligocene interval. Among these climatic trends, the cooling event at ~39 Ma (Cooling Event B) is recorded here for the first time. Variations in fine-fraction d18O values at Site 748 are associated with changes in the Twwt index, supporting the idea that significant short-term variability in surface-water conditions occurred in the Kerguelen Plateau area during the middle and late Eocene. Furthermore, ODP Site 748 calcareous nannofossil paleoecology confirms the utility of these microfossils for biostratigraphic, paleoclimatic, and paleoceanographic reconstructions at Southern Ocean sites during the Paleogene.

Radar profiles from South Shetland Islands/Western Antarctic Peninsula

The sedimentary architecture of polar gravel-beach ridges is presented and it is shown that ridge internal geometries reflect past wave-climate conditions. Ground-penetrating radar (GPR) data obtained along the coasts of Potter Peninsula (King George Island) show that beach ridges unconformably overlie the prograding strand plain. Development of individual ridges is seen to result from multiple storms in periods of increased storm-wave impact on the coast. Strand-plain progradation, by contrast, is the result of swash sedimentation at the beach-face under persistent calm conditions. The sedimentary architecture of beach ridges in sheltered parts of the coast is characterized by seaward-dipping prograding beds, being the result of swash deposition under stormy conditions, or aggrading beds formed by wave overtopping. By contrast, ridges exposed to high-energy waves are composed of seaward- as well as landward-dipping strata, bundled by numerous erosional unconformities. These erosional unconformities are the result of sediment starvation or partial reworking of ridge material during exceptional strong storms. The number of individual ridges which are preserved from a given time interval varies along the coast depending on the morphodynamic setting: sheltered coasts are characterized by numerous small ridges, whereas fewer but larger ridges develop on exposed beaches. The frequency of ridge building ranges from decades in the low-energy settings up to 1600 years under high-energy conditions. Beach ridges in the study area cluster at 9.5, 7.5, 5.5, and below 3.5 m above the present-day storm beach. Based on radiocarbon data, this is interpreted to reflect distinct periods of increased storminess and/or shortened annual sea-ice coverage in the area of the South Shetland Islands for the times around 4.3, c. 3.1, 1.9 ka cal BP, and after 0.65 ka cal BP. Ages further indicate that even ridges at higher elevations can be subject to later reactivation and reworking. A careful investigation of the stratigraphic architecture is therefore essential prior to sampling for dating purposes.

Age determination and stable carbon isotope ratios of Cassidulina laevigata of sediment core Svanic90_9004

A well-dated high-resolution d13C record of the last 2400 a, based on the benthic foraminifera Cassidulina laevigata, is presented for Gullmar Fjord, Sweden. The time interval covers die Roman Warm Period (RWP), the Viking Age/Medieval Warm Period (VA/MWP), the little Ice Age (LIA) and the most recent warming. There is little variation in the d13C record until the early Viking Age (AD 800), when the d13C signal becomes significantly more negative and continues to decrease throughout the VA/MWP, The d13C signal increases both at the beginning and at the end of the LIA but is marked by more negative values during the larger part of the period. Since about 1970, the d13C values are more negative than the long-term average. This general negativity of the record may result from a higher flux of organic matter, possibly of terrestrial origin due to land-use changes together with moderate changes in stagnation periods since the VA/MWP. In most recent times, the oceanic Suess effect together with increased number of extended stagnation periods are probably the main causes of the shift towards more negative d13C values.

A magneto-mineralogical study of basalt at DSDP Leg 73 Holes

The basalts in Holes 519A, 522B, and 524 were studied for intensity of natural remanent magnetization, magnetic hysteresis, magnetic susceptibility, stability of isothermal remanence, and thermomagnetic behavior. Some of these properties are sensitive to both the composition and the microstructure of the magnetic minerals, others to composition only. Thus it is possible to separate the two effects and to trace the variation of effective magnetic grain size and degree of alteration within a lithologic unit or over a yet larger distance or time interval. The flow in Hole 519A is highly maghemitized at the top, the degree of maghemitization decreasing with depth in the flow. Effective grain size increases with increasing depth. Electron microprobe analysis of the titanomaghemite grains in these samples provides no support for the leaching out of iron during alteration. The pillows and flows in Hole 522B are distributed among a number of cooling units, and no systematic downhole variations are apparent. The inferred magneto-petrology is consistent with the cooling and alteration history that might be expected within the units. The upper and lower sills in Hole 524 are more uniform and have a larger concentration of well-developed magnetic mineral grains than the pillows and flows in Holes 519A and 522B. Maghemitization appears to have developed from the boundaries of the sills that are in contact with the sediments between the sills.

Late Miocene carbon isotope and paleoproductivity record

We examine whether or not a relationship exists between the late Miocene carbon isotope shift (~7.6-6.6 Ma) and marine productivity at four sites from the Indian and Pacific Oceans (Ocean Drilling Program Sites 721, 1146, 1172, and 846). We use a multiproxy approach based on benthic foraminiferal accumulation rates, elemental ratios, and dissolution indices, and we compare these data to benthic foraminiferal d13C values measured on the same samples. Although some of these sites have been targeted previously in studies of either the late Miocene/early Pliocene "biogenic bloom" (Sites 721 and 846) or the late Miocene carbon isotope shift (Site 1172), our records are the first to establish paired proxy records of carbon isotopes and paleoproductivity allowing a direct assessment of a potential link. Our results indicate that at all sites, productivity increased sometime during the d13C shift; at three sites (721, 1146, and 846), productivity increased at the beginning of the shift. The correlation coefficients derived from linear regression between micropaleontologically derived productivity and foraminiferal d13C values are relatively high during the time interval containing the late Miocene d13C shift (and statistically significant at three of the sites). Carbon flux and isotope mass balance considerations illustrate that transfer of organic matter between the terrestrial and marine reservoirs together with enhanced oceanic upwelling best approximates observed changes in carbon isotope records and paleoproductivity. We note that long-term trend in the Site 846 paleoproductivity record can be correlated to the long-term trend in the Site 848 eolian flux reconstructions of Hovan (1995, doi:10.2973/odp.proc.sr.138.132.1995) hinting at a link between strengthened wind regime and productivity during the late Miocene.

Stable isotope record and age determination of ODP Site 121-758 in the northeastern Indian Ocean

Numerous studies have shown that delta18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous delta18O and delta13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991, doi:10.2973/odp.proc.sr.121.124.1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic delta18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980, doi:10.1126/science.207.4434.943). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991, doi:10.1016/0012-821X(91)90206-W, 1991, doi:10.1016/0012-821X(91)90082-S), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of delta18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0-3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100-, 41- and 23-kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.