PP23A-1456
Early Deglacial Changes in North Pacific Surface and Deep-Water Circulation -- the Record of C14 Reservoir Ages and Sea Surface Salinities
Near the end of the Last Glacial Maximum, deep and intermediate waters in the subarctic North Pacific had apparent ventilation ages of 3300-3600 yr. 14C reservoir ages of surface waters varied from 1200-1700 yr, if accepting the results of the 14C plateau tuning method (Sarnthein et al., 2007) instead of assuming a constant glacial average value of 570 yr (Broecker and Barker, 2007). With the onset of H1 near 17,500 cal. yr BP (GISP2 record of Meese and Sowers, 1997; Hulu Cave record of Wang et al., 2001) surface water ages in the subarctic northwest Pacific started to drop to 300 yr. This low value suggests the onset of a phase of deep-water convection in the northwest Pacific, which is also reflected by an abrupt 1200-to-1500-yr drop in the ages of both upper deep waters (2300 m w.d.) in the subarctic northwest Pacific and lower deep waters (3600 m w.d.) in the northeast Pacific. The deep penetrating circulation change implies that major portions of the North Pacific Ocean underwent a significant increase in deep and intermediate-water ventilation over H1 times, which differs from previous assumptions. This may have great implications for the "Conveyor Belt" concept and a better understanding of a coeval 190-permil shift in atmospheric 14C. The onset of deep-water formation near 17.5 ka may be linked to a major salinity increase in the subarctic northwest Pacific (up to 36 psu), which in turn may result from a reduced freshwater supply by the Kuroshio Current which collects all major rivers of Southeast Asia and feeds the Pacific subpolar gyre like a rain gully. This freshwater supply strongly decreased around 17.5 ka with the abrupt start of severe Southeast Asian aridity, as documented in the Hulu Cave. Broecker and Barker, 2007, EPSL 256, 90-99. Meese and Sowers, 1997, JGR 102, 26,411-26,423. Sarnthein et al., 2007, AGU Monograph 173, 175-198. Wang et al., 2001, Science 294, 2345-2348.
PP23A-1457
North Pacific Deep Water Formation and Carbon Dioxide Rise During the Last Deglaciation
Mechanism for abrupt atmospheric carbon dioxide increase during the last deglaciation associated with global overturning circulation remains uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep water during the glacial periods, but direct evidence regarding deglacial ventilation changes in the mid-deep depth North Pacific water is sparse and often equivocal. Here for the first time we present new evidence for such ventilation changes from sediment sequences from the NW Pacific close to North Pacific intermediate water (NPIW) source regions. Radiocarbon age differences between coexisting planktic and benthic foraminifera are presented for paired well dated mid and deep depth sediment sequences as a measure of changing ventilation. These records exhibit a clear antiphase relation during the deglacial between North Pacific mid-deep water ventilation and that related to the Atlantic Meridional Overturning Circulation. The North Pacific mid-deep water ventilation rates increased during Heinrich 1 and Younger Dryas cooling episodes and significantly decreased during the warm episodes of the Bølling- Ållerød and preBoreal early Holocene, in response to teleconnected atmospheric changes, reorganization of oceans' overturning circulation and/or bipolar seesaw. During the early deglaciation, the North Pacific may have acted as a major source for atmospheric CO2 rise through newly emerging deep water formation as a result of strengthened NPIW ventilation
PP23A-1458
Deglacial variations in South Atlantic deep water ventilation
New benthic-planktonic foraminiferal 14C results from the deep South Atlantic reveal significant variability in deep water ventilation during the last deglacial period. When combined with other proxies from the same core (carbonate preservation, benthic δ13C, Nd isotopes etc.) our new deep water results from core TNO57-21 (41.1° S, 7.8° E, 4981m) reflect the alternating influences of old, corrosive waters originating from within the Southern Ocean during the 'Mystery Interval' (17.5-14.5 kyr ago) versus newly ventilated waters originating in the North Atlantic during the Bolling/Allerod. Our results suggest very rapid (<500 yr) ventilation of the deep South Atlantic during the B/A. This is also reflected by greatly enhanced carbonate preservation during the same interval.
PP23A-1459
Holocene Radiocarbon Dates on Coexisting Benthic and Planktic Foraminifera Shells in the Subarctic North Pacific
The high North Pacific contains some of the oldest and CO2-rich deep waters in the world; however, variations of the interactions between the surface and subsurface reservoirs and the ice-age history of both land and ocean remain poorly known. Here we present paired planktic and benthic foraminifera radiocarbon dates from jumbo piston cores taken on the Gulf of Alaska margin. The present focus of this research is core EW0408-85JC (59.56° N, 144.15° W, 682 m depth), located on the shelf break at a depth shallow enough to avoid carbonate dissolution, yet still experiencing oceanic conditions as confirmed by the oxygen isotope record. The core extends back >17,000 ybp; sedimentation rates implied by the high- resolution dating vary from an early Holocene low of ~20 cm/ka, to late glacial values of >500 cm/ka (likely associated with rapid flux of glacial silt to the ocean). Regional retreat of glaciers behind fjord sills is constrained to >16 ka (calendar). Benthic-planktic 14C age differences (B-P) mostly track values expected from changing atmospheric production rates as they propagate into the ocean system, with a modern B-P of 500 yr. Exceptions include high B-P values of ~800 yr in mid-Holocene time. This interval is associated with reduced percentages of the foraminiferal species G. bulloides, implying greater upper- ocean stratification or weaker upwelling than at present. High B-P values during glaciation, up to ~1000 yr, may reflect the presence of relatively old subsurface waters (i.e. no apparent increase in shallow subsurface ventilation by intermediate waters during glacial time in this region), and/or increased stratification associated with freshwater inputs from regional tidewater glaciers.
PP23A-1460
Variations in the Reservoir Age of the NE Pacific Over 6,000 Years Suggest Changes in Ocean Circulation Linked to Climate
The difference between the 14C ages of closely-paired marine and terrestrial material reflects the transport of "older" 14C�depleted subsurface waters to the surface and is a measure of regional or secular ocean circulation (reservoir age). After correction for known variations in atmospheric 14C levels, variation in reservoir age of the ocean at a single site over time may indicate changes in ocean circulation. Long-term data from single or closely associated sites in the NE Pacific are rare. We obtained data from Mink Island, an archeological site in Katmai National Park and Preserve, located in the Shelikof Strait in the northern Gulf of Alaska. We compare 14C dates of paired marine and terrestrial material selected from different climate periods over the last 6,000 years. The dated terrestrial material is charcoal from human occupation surfaces, while the marine dates are from midden shells of the bivalve, Saxidomus giganteus, surveyed at the same excavation level. A rigorous examination of the archeological context is being used to validate the pairings. We have found some deviations from expected values of reservoir age over time, which suggest changes in ocean circulation over the last 6,000 years, likely in response to climate.
PP23A-1461
Decreased Variability in Shallow Water Ventilation in the North Atlantic during the End of the Little Ice Age
Previous work examining corals from the North Atlantic employed the ratio of 14C/12C to document that large decadal pulses in North Atlantic ventilation occurred from 1890 to 1950 (Druffel 1997 and 1989). The results of these studies revealed variable reservoir age corrections for the North Atlantic surface ocean and drew connections between deep ocean circulation and the North Atlantic Oscillation (NAO). Being able to extend this record further back in time can provide insight into whether this behavior occurred during the end of the Little Ice Age (1800-1850) and the Dalton solar minimum (1800-1830). In this study, we measured radiocarbon from a brain coral (Diploria labyrinthiformis) collected along the south shore of Bermuda. This colony lived from 1780-1999, capturing both recent and historical periods of interest. Biennial Delta 14C measurements from 1893-1921 show approximately 70 percent less variability than results previously reported from the same species collected along the north shore (p=0.01, n=15). Low variability along the south shore is also seen during the end of the Little Ice Age (LIA), implying that pulses of rapid ventilation seen on the north shore of Bermuda are not as clearly defined to the south. The absence of a significant correlation between del18O and Delta14C (r=0.31, p=0.22, n=17), which was previously found in the corals to the north of Bermuda, indicate less of an influence of deep water mixing. These results raise questions about circulation of ocean waters around the island of Bermuda and the Sargasso Sea. Delta 14C values from the south shore, however, do not show a difference in the mean value during the end of the LIA relative to the beginning of the 20th century. Samples measured from 1800-1840 (n=6) have an average of approximately 2 permil higher than samples taken after 1840 (n=21), a difference that is within the range of error (�2.5-4.0 permil). During the Dalton solar minimum we would expect higher Delta 14C values in atmospheric CO2 due to increased stratospheric production of 14C. The lack of a change in surface water 14C may indicate increased vertical mixing at this time, consistent with colder and less saline sea surface water (Goodkin et al. 2008) at this site and an extended positive NAO (Luterbacher et al. 2001).
PP23A-1462
A Rapid Radiocarbon Method for Age Surveys of Southern Ocean Deep-sea Corals
Deep-sea corals provide a unique archive of past ocean radiocarbon because they are sessile and can be dated independently using U-series nuclides. One difficulty, however, is that using current techniques it is impractical to date large numbers of corals in order to determine which specimens have the appropriate ages for radiocarbon reconstructions. Here we present results from a quick method of making graphite for radiocarbon dating that reduces the amount of sample preparation time, thus allowing us to date a greater number of corals. In addition, these rapid age surveys provide important information on coral age populations, allowing us to examine coral distributions through time. The corals used in this study come from a sample set of about 6,000 specimens of Flabellum, Balanophyllia and Desmophyllum spp. collected from the Drake Passage area (50S -70S, 120 m-1700 m depth). Replicate samples from a single coral yielded a standard deviation of 81 years (n=9). Variations in sample mass (3 to 85 mg) have no clear effect on the Fm and furthermore, a simple cleaning using methanol yields the same results as a more involved cleaning procedure that includes an oxidizing solution and perchloric acid rinse. To improve the efficiency of the method, we assumed a delta13C = 0 per mil. This assumption is likely our largest source of uncertainty, resulting in offsets of up to 200 radiocarbon years over a reasonable range of delta13C. This level of uncertainty is sufficiently low to allow distinction between corals from different time periods over the past 35 ky (e.g. Last Glacial Maximum, Younger Dryas, etc.). To date, we have found corals from Burdwood Bank dating from the modern to the Younger Dryas and corals from the Drake Passage dating from the modern to Heinrich Event 1, which will be used in future paleo-climatic reconstructions in this important part of the ocean.
PP23A-1463
Calibration of fossil scleraxonian Southern Ocean deep-sea corals for U-series dating
The deep Southern Ocean has been pinpointed as candidate reservoir capable of storing the additional respired carbon that was drawn from the atmosphere during the Last Glacial Maximum compared with the present-day. In this context the determination of deep ocean ventilation ages is a commonly applied tool, potentially identifying radiocarbon depletion in glacial deep water and enhanced ocean stratification. In order to derive deep-sea ventilation ages most studies to date have used either radiocarbon age differences between paired planktic and benthic foraminifera samples or coupled U-Th and radiocarbon dates obtained from aragonitic deep-sea corals. Results from both these approaches are, however, as yet very scarce for the Southern Ocean. We present calendar ages for a set of deep-sea scleraxonian corals from the Marie Byrd Seamounts in the Amundsen Sea sector of the Southern Ocean (~123°W, ~69°S, 2500 m to 1430 m water depth) employing the 230Th/U-dating method. The aim of our study is to evaluate whether these calcitic octocorals can be used for ventilation age determinations. Our corals have significantly lower uranium concentrations than aragonitic deep-sea corals, ranging from 80 to 250 ng/g. Most corals of Holocene age reproduced the present-day seawater 234U/238U. Pre-Holocene corals, however, show a systematic enrichment of 234U, leading to slightly elevated deglacial initial 234U/238U and significantly higher 234U/238U for ~MIS5 sub-samples. These corals also appear to grow very slowly, on the order of only few μm/year, making it essential to sample as little coral material as possible for combined 230Th/U- and radiocarbon dating purposes. One coral, sampled at high-resolution in various sections returned ages that scatter around 10 ka BP and the early deglaciation, though several significantly older ages were obtained as well. The present-day (234U/238U) ACT in different sections of this coral is very homogenous (1.155 ± 0.003) and more or less independent of apparent age, suggesting effective preferential 234U diffusion throughout the coral. Strikingly this composition is also higher than the present-day seawater (234U/238U) ACT of 1.147. We show the apparent 230Th/U coral ages and 234U/238U results alongside coral major and trace element ratios obtained by laser ablation along selected transects, to better characterize U diffusion pathways and mechanisms, to identify internal diagenetic alteration rims and to evaluate the presence of coral paleo- surfaces.
PP23A-1464
Benthic Foraminifer Nd Isotopes and Radiocarbon: a Preliminary Study
The combined analysis of neodymium (Nd) isotopes and radiocarbon (14C) in sedimentary benthic foraminifera has the potential to remedy a major issue involved in the interpretation of 14C data. The problem is that changes in deep-water sourcing, deep-water source signature and deep-water renewal rates may all affect 14C concentration, complicating the interpretation of deep-water 14C reconstructions. To overcome this, the application of a circulation tracer is required. Traditional proxies such as δ13C, Cd/Ca, and deep-water temperature either do not behave as conservative tracers or only indirectly represent water mass mixing, and so are of limited use1. The application of a more recently developed proxy for water mass sourcing, circulation and mixing, Nd isotopes, might offer a solution to this problem. A recent pioneering study2 has demonstrated that sedimentary benthic foraminifera might be a suitable archive of past deep-water Nd isotope composition at high resolution, potentially more robust than acidic-reductive sediment leach data. This study seeks to utilise this promising new tracer by applying it to an investigation of deglacial ventilation changes in the North Atlantic. Initial data presented here for core-top benthic foraminifera from the Labrador Sea, Iberian Margin and Pacific illustrate nearly the complete dynamic range of Nd isotope compositions present in the modern ocean, in general agreement with existing seawater Nd isotope data. Preliminary down-core Nd isotope data from the Iberian Margin place initial constraints on the contribution of deep-water sourcing to observed changes in deep-water 14C ventilation across the last deglaciation. 1 - E. Boyle, Ann. Rev. Earth Planet. Sci. 20, 245-287 (1992) 2 - V. Klevenz et al., Earth Planet. Sci. Lett. 265, 571-587 (2008)
PP23A-1465
Compilation of Marine Radiocarbon Bomb-Pulse from the Temperate North Atlantic Using Annually-Resolved Time-Series From Arctica islandica
Radiocarbon measurements from increments of annually-banded corals covering the past 60 years from sub- tropical and tropical contexts provide valuable records of the marine expression of the atmospheric excess radiocarbon “bomb-pulse”. These records can be used as calibration series for high-resolution post- bomb radiocarbon dating and constitute tracers for identifying watermass age and mixing processes. Hitherto such applications have been restricted in temperate shelf seas because of the lack of widespread measurements from annually-resolved archives. Here we present a compilation of bomb-pulse data from annual growth increments of the shallow marine bivalve mollusc Arctica islandica from sites across the temperate North Atlantic (Georges Bank, north Icelandic shelf, north Norway, North Sea). The temporal response is highly correlated at all sites, but the amplitude of the bomb-pulse varies, with the highest values attained in the North Sea and the most damped response on the north Icelandic shelf. These differences can be attributed to the integrated hydrographic context of these sites (entrainment of deep, old water; rates of air-sea exchange; fluvial runoff). The north Icelandic data contain a reversal in the rising limb of the bomb- pulse which is not present elsewhere, even in the more sensitive sites. This reversal correlates with instrumental data characterising the Great Salinity Anomaly of the 1960s when old (deltaR = + 200 years), cold and relatively fresh East Icelandic Current flooded the north Icelandic shelf as a result of southward migration of the Polar Front. The bomb-pulse radiocarbon proxy is therefore a sensitive proxy for hydrographic variability. Further applications of these data will be discussed.
PP23A-1466
Dating Taal Lake sediment of Philippines by using the bomb radiocarbon curve
Taal Lake is located in Batangas Province of central Philippines (14° 00.1'N, 120° 59.1'E), with a surface area of 267 km2, a maximum depth of 176 m and an elevation of 3 m above sea level. The lake occupies the famous Taal Volcano system which consists of a 15 × 22 km prehistoric caldera. The 5 km wide Taal Volcano Island which has 47 craters and 4 maars, lies in the north central Taal Lake. With 34 recorded eruptions, Taal Volcano is considered one of the most active volcanic centers in the Philippines and is one of the 16 monitored volcanoes by the Global Volcanism Network. In order to investigate specifically geochemical and isotopic proxies which have paleo-eruption and paleoclimate significances in the sediment cores, we had retrieved two gravity cores from the lake in 2007: TLS1 with 90cm length from a water depth of 100m, and TLS2 with 120cm length from a water depth of 15m. Core TLS2 contains plant remains at 16 horizons which were selected for AMS 14C dating at UCI. Only one of the sixteen plant samples has a δ13C value of -16.4 per mil, whereas the rest samples have the δ13C values ranging from -24.2 to -30.1 per mil. The δ13C values indicate that the plant remains are mainly terrestrial C3 plants. The comparisons of Acid-Base-Acid (ABA) treated samples with non-ABA treated samples show the same results within uncertainty. Except for the sample at 16.7cm, the Δ14C of samples from 0 to 95cm are 51.9~401.9 per mil peaked at 79cm depth, matching very well with the bomb 14C curve determined in tree rings. The distribution of the bomb 14C profile allows us to establish the chronology of Core TLS2, which yields a constant sedimentation rate of 2.04cm/year. Based on this chronology, living plants around Taal Lake may have a lower initial Δ14C (-30 per mil) compared to the Modern Standard. Core TLS1 contains much less plant remains. Three samples from depths of 1.2, 38.5 and 72 cm have Δ14C values of -32.9, 9.9 and -141.8 per mil, respectively. This deeper core has much lower sedimentation rate, spanning about the last one thousand years. With core materials, we are able to uncover the sedimentary history of the lake.
PP23A-1467
Radiocarbon calibration between 30 and 45 ka: Evidence from laminated sediments from Japan Sea
During the last glacial period, Japan Sea is a semi-enclosed basin and laminated sediments were formed under dissolved O2 deficient environment in the bottom water. In a sediment core collected from northeastern Japan Sea (KY04-09), we have observed a down-core trend of total organic carbon (TOC) content quite similar with oxygen isotope record of Greenland ice cores during MIS-3. Although the mechanism connecting between these two parameters is still unclear, climate over Greenland was linked with surface ocean environment in Japan Sea through the atmosphere (Yokoyama et al., 2006). We determined radiocarbon content in planktonic foraminifera Globigerina bulloides from 28 sections in the sediments. Visual comparison between oxygen isotope records of Greenland ice cores and TOC in KY04-09 provides a unique calibration of the radiocarbon time scale between 30 and 45 ka. It suggests radiocarbon production rate in the atmosphere was substantially elevated around 40 cal. ka B.P. which is associated with Laschamp geomagnetic field minimum. This result is basically consistent with the one estimated from Cariaco Basin sediments reported by Hughen et al. (2004).
PP23A-1468
Nd Isotopic Evidence of Deglacial Antarctic Intermediate
Nd isotopes from fossil fish teeth and debris are a potential water mass tracer. Initial Nd isotopic analyses from marine sediment core (MC19/GC31/PC08) from a water depth of 705 m on the open margin off the western coast of southern Baja California have been used to test changes in deep ocean circulation proposed by Marchitto et al. (2007). These authors identified two pulses of highly negative Δ14C during the deglaciation from 19 to 11 ky at this shallow water site. They proposed that the negative Δ14C signal represented a water mass that was isolated from the atmosphere for a prolonged period of time. Specifically, they suggested that extensive sea ice growth off Antarctica during the last glacial period reduced atmospheric exchange with the deep ocean and created conditions favorable for brine formation, thereby redistributing CO2 from the atmosphere to the deep ocean. Deglaciation and retreat of the sea ice would have re-established ventilation of the Southern Ocean and returned old carbon to the sea surface and atmosphere. Pulses of old, poorly ventilated, negative Δ14C water observed at site MC19/GC31/PC08 suggest renewed Southern Ocean upwelling and spreading of the signal via Antarctic Intermediate Water (AAIW), coincident with the timing of the atmospheric CO2 rise across the last deglaciation. This interpretation requires that AAIW was volumetrically more important along the Baja California margin during deglaciation than it is today. While radiocarbon data indicate the age of the water mass at this shallow site, Nd isotopic data can be used to determine the source of this water. Nd isotopic values of 0 to 2 ε Nd units after the deglaciation from the Baja site are similar to values for Pacific intermediate waters. In contrast, values of -2 during the deglaciation trend towards typical AAIW values, supporting the interpretation of deglacial circulation and ocean/atmosphere CO2 exchange proposed by Marchitto et al. (2007). Additional Nd isotopic work may allow us to estimate the percent contribution of AAIW before, during, and after deglaciation, and thereby enhance our understanding of the deep ocean circulation in the North Pacific and carbon redistribution during the deglaciation. Reference: Marchitto, T.M., S.J. Lehman, J.D. Ortiz, J. Fluckiger, and A. van Geen, Marine Radiocarbon Evidence for the Mechanism of Deglacial Atmospheric CO2 Rise 10.1126/science.1138679, Science, 316 (5830), 1456- 1459, 2007.