Millennial-scale paleoceanographic features in the central Bering Sea during the last 65 kyrs
An 18-m long piston core (PC23A) was collected from the northern slope area (60°09.52'N, 179°27.82'W, 1002 m deep) in the Bering Sea during MR06-04 cruise by R/V Mirai. The age of the upper part of core was determined by planktonic foraminifera AMS 14C dates. The presence of L. nipponica sakaii and absence of A. setosa constrained the age of lowermost of core to be younger than 65 kyr. The sedimentation rate (up to 30 cmkyr-1) of core PC23A is enough to record the short-term paleoclimatic events. The overall CaCO3 contents are generally low (less than 10%) and lower than opal contents (less than 20%), confirming that the modern Bering Sea is typical of a silica ocean. The downcore variation of CaCO3 and biogenic opal contents shows not only clear orbital (glacial-interglacial) change throughout the core but also the distinct millennial-scale signals during the last glacial period. For example, a series of prominent CaCO3 peaks during the last glacial period can be clearly correlated with Dansgaard- Oeschger (D-O) events. These increased CaCO3 peaks are attributed either to the enhanced productivity, causing the expansion or migration of oxygen minimum zone or to the reduction in intermediate-water ventilation of the North Pacific, enhancing degree of CaCO3 preservation potential. The high geochemical (CaCO3, biogenic opal, and TOC) values in the upper part of core correspond to the deglacial laminated sediments. At this interval, Fragilariopsis cylindrus (sea-ice indicator diatom) and Rhizoplegma boreale (cold and low saline water indicator radiolarian) rapidly increased. These results indicate that the high primary production during deglaciation was caused by the ice-edge bloom which occurred in surface meltwater layer in the early spring, and river-water discharge played an importance role in nutrient supply during this time. Increase in TOC contents at the end of LGM may be attributed to climate amelioration which favored more coccolithphorids than diatoms.
Holocene Intermediate Water Variability in the Northeast Pacific: A Santa Barbara Basin 14C Record
Pairs of benthic and planktonic foraminiferal 14C dates from the Santa Barbara Basin, offshore southern California, are used to assess millennial-scale hydrographic changes in the northeast Pacific during the Holocene. Current models of North Pacific Intermediate Water formation during the last glacial and the Holocene vary widely in their predicted relationships between thermal conditions in the north Pacific, 14C age and oxygen content of the Santa Barbara Basin's sub-sill bottom waters, and productivity in the basin's surface waters. Using benthic-planktonic 14C age differences as a proxy for North Pacific Intermediate Water influence on the basin's bottom waters and biogenic silica and sedimentary bioturbation as records of past productivity and bottom-water oxygenation, we test our data against these various models. It appears that the different models can be reconciled by invoking bifurcation of eastward-moving intermediate water along the North American coast: Changes in the benthic-planktonic 14C age difference and other proxies in Santa Barbara Basin can be explained by movement of the bifurcation point to the north or south during warm and cool intervals of the Holocene. Inferred behavior of water at intermediate depths on the northeast Pacific margin is compared with surface conditions and with records of deep circulation from the North Atlantic, with the Santa Barbara Basin records supporting surface and intermediate connection between the two ocean basins at millennial time scales during the Holocene.
Cyclicity of Wind-Driven Upwelling in the Southern Gulf of California during the Middle and Late Holocene
The radiolarian fauna and magnetic susceptibility were analyzed in a sequence of laminated sediments from the Alfonso Basin, Gulf of California as proxies of the climatic and oceanographic conditions during the past 8000 years. A factor analysis from 212 continuous samples identified three assemblages. One of them, Factor 3, denominated by Perez-Cruz in 2006 "Cooling events", records several episodic events throughout the core. The radiolarian assemblage is characterized by species that reflect ocean surface and cooling conditions, and most likely, upwelling processes. Spectral analyses were performed on factor 3 and magnetic susceptibility time series. The results show regular 500 year climate cycles linked of combined forcing mechanism. Changes in circulation wind pattern in North America promoted strong northwesterly winds dominating the Gulf of California. This wind regime causes intense upwelling along the axis of the Gulf, including Alfonso Basin, and colder, nutrient reach subtropical subsurface water (SSW) is brought to the surface by displacement of the warmer equatorial surface water (ESW).
Evaluating Sea Surface Temperature Variability Using Replicated Porites lobata Coral Sr/Ca Records From Clipperton Atoll (1994-1894AD)
The fidelity of coral Sr/Ca as a tracer of water temperature was assessed at monthly resolution in cores from three coral colonies of Porites lobata at Clipperton Atoll (10° 18'N, 109° 13'W) in the eastern Pacific. Significant linear relationships were found between individual cores and instrumental sea surface temperature (SST) for the 20-year calibration period of 1994-1974 (R2= 0.55, 0.52, and 0.49; P < 0.001) with similar slopes ranging from 0.065 to 0.069 mmol/mol/° C but with slightly different y-intercept values (11.19, 11.02, and 11.15). The 3-core composite average increased the regression to instrumental SST (R2= 0.66) with a slope of 0.068 mmol/mol/° C and y-intercept of 11.12. Correlations of monthly Sr/Ca data between cores was high (core 2B vs. 6A from 1994 to 1942, R= 0.69; core 2B vs. 4B from 1994 to ~1934, R=0.44) with the exception of a 10-year section of growth in one coral. SEM images and skeletal δ18O and δ13C revealed nothing anomalous about this interval. Inter-colony comparison of calcification rates displayed poor reproducibility and does not appear to exert a significant influence on skeletal Sr/Ca. The composite Clipperton Sr/Ca record (without data from the anomalous interval) exhibits interannual fluctuations highly correlated with the El Niño Southern Oscillation (ENSO) that are in phase with observed gridded SST record (Global Ice and Sea Surface Temperature Ver. 2.2 (GISST)). Despite the greater amplitude of annual and interannual variability in the composite Sr/Ca-SST record relative to gridded SST, the overall similar ENSO frequency and periodicity attests to the high degree of reproducibility of this temperature proxy at Clipperton. Our coral derived SST record shows dampened ENSO-band variability in the 1920s-1940s during the well- documented "ENSO quiet period". Examination of only the boreal spring data in the composite Sr/Ca record and in Clipperton δ18Oseawater record (using coral skeletal δ18O and Sr/Ca data from 1994-1894) indicates reduced interannual spring SST variability and greater interannual δ18Oseawater variability from ∼1925 to mid 1940s. These observations support change in freshwater flux in the region and imply anomalous variations in the Intertropical Convergence Zone during the ENSO quite period. An observed warming trend of ∼1°C since 1976 may have been related to the 1976 Cold Event and subsequent interdecadal climate shift of the Pacific Ocean. These results highlight the need to replicate records in coral-based paleoclimatology, but also support the conclusion reached by many studies for SST driven Sr/Ca variability in rapidly growing Porites corals.
High-resolution Analysis of Trace Elements in Encrusting Coralline Red Algae by Laser Ablation ICP-MS
Coralline red algae constitute an ideal biogenic marine climate recorder owing to their common occurrence in mid- to high latitude oceans and their continuous growth. Encrusting coralline red algae have great potential as paleoclimate archives because they deposit spatially fixed annual growth increments in a high Mg-calcite skeleton and can reach ages of up to several hundred years. Here we present high-resolution Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) trace elemental analyses (Mg, Sr, Ba, U) from several coralline red algal specimens of the genus Clathromorphum, collected from the North Atlantic and North Pacific Oceans, that display average growth rates of around 300µm/year. Elemental ratios (Mg/Ca, Sr/Ca, Ba/Ca, U/Ca) were measured in sub-monthly resolution for up to century-long segments of coralline red algal growth. Several overlapping transects were analyzed in order to assess the robustness of the proxy data. The reproducibility is excellent and LA-ICP-MS measured Mg/Ca ratios were validated by comparison to electron microprobe data. In addition, data accuracy was tested by comparison to solution ICP-OES data from a bulk sample manually removed parallel to the laser ablation and electron microprobe transects. In particular, algal Mg/Ca ratios show a high degree of correlation with local seawater temperature on different timescales, providing further evidence for the temperature dependency of algal Mg/Ca variations and their use as a valuable paleothermometer. Hence, this study demonstrates the feasibility of extracting high-resolution geochemical signals from encrusting coralline red algae (such as Clathromorphum sp.) using laser ablation ICP-MS. This analysis technique allows rapid continuous sampling of the algal surface with unprecedented resolution and provides a valuable tool for future analysis of algal-derived environmental records.
Using Tree-Rings and Lake Sediments to Infer Historical Water Availability in Jasper National Park, Alberta, Canada
The headwaters of the Athabasca River in Jasper National Park may experience large-scale, persistent droughts that can have significant economic and environmental impacts downstream for the Oil Sands developments and other water stakeholders. Unfortunately, it is impossible to document the full range of natural climate variation or determine the frequency, magnitude or duration of large-scale drought from the few short climate records available for this area. This research will create two independent drought records using a network of 22 annually resolved, precipitation sensitive Douglas fir (Pseudotsuga menziesii) tree-ring chronologies and three 2000-year long high-resolution (one sample per 5-10 years) lake records from the headwaters of the Athabasca River. This paper will present the preliminary results from 7 new tree-ring sites and discuss some of the early limitations in the diatom analyses from the lake sites. It is hoped that this multi- proxy approach to paleo-climate reconstruction will maximize resolution and allow for mutual verification, creating a higher degree of confidence and a more robust picture of natural climate variability than either proxy alone. The resulting reconstructions will document long-term water availability in the headwaters over a 1- 2000 year period for an area where downstream industrial demands for water have drastically increased and where promises of future water allocations have been determined based on a short instrumental record.
A non-invasive multiproxy approach to recognize Holocene paleocoastal environmental signals in Sri Lanka.
Coastal lagoons are archives of paleocoastal environmental signatures. Lagoonal cores are extensively used to recognize paleo-sea level changes, plaeoclimatic changes, paleo-tsunami and storm deposits. Grain size, microfossil assemblages and organic carbon content are some of the common proxies used in such paleoenvironmental studies. This study attempts to use petrophysical methods to measure the physical properties of lagoonal cores to recognize paleoenvironmental signatures. Three sediment cores, each five meters in length, were collected in a 1 km long transect from a siliciclastic coastal lagoon at Kirinda, Sri Lanka. This south-eastern lagoon is highly susceptible to tsunamis and coastal flood events; The 2004 Asian tsunami generated 7-8 m waves in the area. Evidence for Holocene sea level changes are also preserved in nearby areas. Particle size, magnetic susceptibility and visible color reflectance were measured in the three cores at 1 cm resolution. Principal component analysis (PCA) was carried out with grain size (Q-mode) and reflectance data (R-mode). Log records and depth variation diagrams of grain size, reflectance factor scores, and magnetic susceptibility were compared to identify paleo-environmental signals. PCA analysis of reflectance data identified three principle components which describe 92% of the variance while a similar analysis performed for grain size data identifies six components describing 98% of the variance. Downcore variation plots show that a*, b* and the reflectance factor scores representing sediment goethite and iron oxide content have a strong correlation with grain size factors representing the medium sand, silt and clay size classes. Sand layers deposited by 2004 tsunami event and by similar older events can be clearly recognized using these parameters. Magnetic susceptibility plots also show peaks in some of the same sand layers indicating the association of magnetic mineral-rich beach sand. Downcore plots of these petrophysical parameters show a significant abrupt change in the signal at about 2 m below the surface. According to an age model constructed for a nearby lagoon by Jackson (2009) this break dates back about 6000 yrs BP. This break may represent the mid Holocene sea level transgression, which resulted in about 1.5 m sea level rise in Sri Lanka (Katupota, 1995) Correlation of multi proxy downcore variation plots from Kirinda lagoon with geomorphologically and geographically different lagoons on the eastern coast would enable distinguishing different coastal events in the Holocene history.
Late Neogene Paleosols and Paleoclimate, Mweiga Area, Mount Kenya, East Africa
A sequence of lavas, principally trachyandesites and tephrites of Pliocene age, forming the underpinning of the Mt. Kenya Volcanic Series and long considered to have a Pliocene/Miocene age, were recently dated by Ar/Ar, and from oldest to youngest, gave ages of 5.48, 5.38 and 5.19 Ma. All dated lavas are reversely magnetized. Outcrops of two flows, both with interbedded paleosols, and overlain with loess occur near the Amboni River north of Mweiga, Kenya, along the Nyeri/Thompson Falls Road at 0 degrees 18 minutes S; 37 degrees 48 minutes E. Mineralogy of lavas was determined using microprobe and whole rock chemistry, while sediments and paleosols were analyzed using X-ray diffraction and Energy-Dispersive Spectrometry. Sediments and paleosols were studied for lithologic uniformity, weathering characteristics, and clay mineralogy. Ages of lavas and sediments were determined by Ar/Ar dating and paleomagnetism. The compound paleosols of Late Neogene age depict a long and complex weathering history of rock alteration, leading in part to the formation of claystone and later saw the addition of loessic sediments. The paleosols appear to have been episodically deflated, initially in phase with outpourings of lavas, and later with periods of climatic deterioration when wind systems intensified. Such disjunct weathering histories within multi- component paleosol profiles are similar to those documented on nearby Mount Kenya, where well-weathered lower paleosol horizons developed on Matuyama-age till are overlain by much younger, less weathered paleosols developed on Brunhes-age loess. These first-reported Ar/Ar ages, and paleomagnetism of lavas within the very Late Miocene/Early Pliocene, provide maximum ages for initial weathering rates in a xeric tropical highland climate. Clay mineral analyses of both the weathered bedrock and surface loessic sediments indicate an initial prolonged dry climate during the Lower Pliocene, followed by more humid conditions during the Upper Pliocene and Quaternary.