Stable isotope ratios measured on molluscan from the CRP sediment cores in the Ross Sea, Antarctica

Stable isotope analyses of marine bivalve growth increment samples have been used to estimate early Oligocene (29.4 - 31.2) Ma and early Miocene (24.0 Ma) seafloor palaeotemperature from the southwestern continental margin of the Ross Sea. Measured d18O values average +2.5 ? in the early Miocene and range between +1.26 to +3.24 ? in the early Oligocene. The results show that palaeoceanographic conditions in McMurdo Sound during the mid-Cenozoic were significantly different from those of today. The minimum estimated spring through late summer seasonal temperature range was 3°C during the early Miocene and between 1 and 5°C during the early Oligocene. This compares to the equivalent modern day range of <0.5°C within the sound. Absolute seawater temperatures at <100 m depth were of the order of 5 to 7°C during both time slices, compared to modern day values of -1.4 to - 1.9°C in the same area. The results are in broad agreement with early Oligocene Mg/Ca temperature estimates from deep Atlantic foraminifera as well as estimates from local terrestrial palynology and palaeobotany.

Sea-surface tempertaure reconstruction of sediment cores from the Southeast Pacific

Applying the alkenone method, we estimated sea-surface temperatures (SSTs) for the past 33 kyr in two marine sediment cores recovered from the continental slope off mid-latitude Chile. The SST record shows an increase of 6.7°C from the last ice age (LIA) to the Holocene climatic optimum, while the temperature contrast between LIA and modern temperatures is only about 3.4°C. The timing and magnitude of the last deglacial warming in the ocean correspond to those observed in South American continental records. According to our SST record, the existence of a Younger Dryas equivalent cooling in the Southeast Pacific is much more uncertain than for the continental climate changes. A warming step of about 2.5°C observed between 8 and 7.5 cal kyr BP may have been linked to the early to mid-Holocene climatic transition (8.2-7.8 cal kyr BP), also described from equatorial Africa and Antarctica. In principal, variations in the latitudinal position of the Southern Pacific Westerlies are considered to be responsible for SST changes in the Peru-Chile current off mid-latitude Chile.

Sea surface temperatures during Marine Isotope Stage (MIS) 11

The Marine Isotope Stage (MIS) 11 (424-374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic (N = 28) or planktonic (N = 31) stable oxygen isotope curves to a common time frame and subjected 48 of them to an empirical orthogonal function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~410 kyr. The second EOF, which explained ~18% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3-6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties >1 °C. In order to validate the CCSM3 (Community Climate System Model, version 3) predictive potential, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Chemical composition of clay minerals from ODP Leg 168 sites

The exchangeable cation compositions of organic-poor terrigenous sediments containing smectite as primary ion exchanger from a series of holes along ODP Leg 168 transect on the eastern flank of the Juan de Fuca Ridge have been examined as a function of distance from the ridge axis and burial depth. The total cation exchange capacity (CEC) values of the sediments ranged from 2 to 59 meq/100 g, increasing with increases in the wt.% smectite. At the seafloor, the exchangeable cation compositions involving Na, K, Mg, and Ca, expressed in terms of equivalent fraction, are nearly constant regardless of the different transect sites: XNa = 0.21 ± 0.04, XK = 0.08 ± 0.01, XMg = 0.33 ± 0.09, and XCa = 0.38 ± 0.09. The calculated selectivity coefficients of the corresponding quaternary exchange reactions, calculated using porewater data, are in log units -5.45 ± 0.39 for Na, 1.97 ± 0.49 for K, 0.42 ± 0.41 for Mg, and 3.06 ± 0.69 for Ca. The exchangeable cation compositions below the seafloor change systematically with distance from the ridge crest and burial depth, conforming to the trends of the same cations in the porewaters. The selectivities for Na and Mg are roughly constant at temperatures from 2 to 66°C, indicating that the equivalent fractions of these two cations are independent of sediment alteration taking place on the ridge flank. Unlike Na and Mg, the temperature influence is significant for K and Ca, with Ca-selectivity decreases being coupled with increases in K-selectivity. Although potentially related to diagenetic and/or hydrothermal mineral precipitation or recrystallization, no evidence of such alteration was detected by XRD and TEM. In sites where upwelling of hydrothermal fluids from basement is occurring, the K-selectivity of the sediment is appreciably higher than at the other sites and corresponds to the formation of (Fe, Mg) rich smectite and zeolites. Our study indicates that local increases in K-selectivity at hydrothermal sites are caused by the formation of these authigenic minerals.

Age model and stable oxygen isotope mean values of ice core SZ99 from Akademii Nauk ice cap on Severnaya Zemlya

Between 1999 and 2001, a 724 m long ice core was drilled on Akademii Nauk, the largest glacier on Severnaya Zemlya, Russian Arctic. The drilling site is located near the summit. The core is characterized by high melt-layer content. The melt layers are caused by melting and even by rain during the summer. We present high-resolution data of density, electrical conductivity (dielectrical profiling), stable water isotopes and melt-layer content for the upper 136 m (120 m w.e.) of the ice core. The dating by isotopic cycles and electrical conductivity peak identification suggests that this core section covers approximately the past 275 years. Singularities of volcanogenic and anthropogenic origin provide well-defined additional time markers. Long-term temperatures inferred from 12 year running mean averages of d18O reach their lowest level in the entire record around 1790. Thereafter the d18O values indicate a continuously increasing mean temperature on the Akademii Nauk ice cap until 1935, interrupted only by minor cooling episodes. The 20th century is found to be the warmest period in this record.