Sachs Julian P.

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Sachs
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Julian P.
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  • Article
    Eastern tropical Pacific hydrologic changes during the past 27,000 years from D/H ratios in alkenones
    (American Geophysical Union, 2007-12-18) Pahnke, Katharina ; Sachs, Julian P. ; Keigwin, Lloyd D. ; Timmermann, Axel ; Xie, Shang-Ping
    The tropical Pacific plays a central role in the climate system by providing large diabatic heating that drives the global atmospheric circulation. Quantifying the role of the tropics in late Pleistocene climate change has been hampered by the paucity of paleoclimate records from this region and the lack of realistic transient climate model simulations covering this period. Here we present records of hydrogen isotope ratios (δD) of alkenones from the Panama Basin off the Colombian coast that document hydrologic changes in equatorial South America and the eastern tropical Pacific over the past 27,000 years (a) and the past 3 centuries in detail. Comparison of alkenone δD values with instrumental records of precipitation over the past ∼100 a suggests that δD can be used as a hydrologic proxy. On long timescales our records indicate reduced rainfall during the last glacial period that can be explained by a southward shift of the mean position of the Intertropical Convergence Zone and an associated reduction of Pacific moisture transport into Colombia. Precipitation increases at ∼17 ka in concert with sea surface temperature (SST) cooling in the North Atlantic and the eastern tropical Pacific. A regional coupled model, forced by negative SST anomalies in the Caribbean, simulates an intensification of northeasterly trade winds across Central America, increased evaporative cooling, and a band of increased rainfall in the northeastern tropical Pacific. These results are consistent with the alkenone SST and δD reconstructions that suggest increasing precipitation and SST cooling at the time of Heinrich event 1.
  • Dataset
    Record of d2H of dinosterol variability in down core lake sediments from Clear Lake, Palau
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2019-08-02) Sachs, Julian P. ; Richey, Julie N.
    Record of d2H of dinosterol variability in down core lake sediments from Clear Lake, Palau. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/699469
  • Dataset
    14C dates from core PC1 collected from T Lake, Palau in September 2013
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2019-06-24) Sachs, Julian P. ; Dawson, Michael N.
    14C dates from core PC1 collected from T Lake, Palau in September 2013 using a Colinvaux‐Vohnout Livingstone‐type rod‐operated piston corer. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/771658
  • Dataset
    Isotopes from B. gymnorhiza mangroves in Palau during 2013
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2021-05-26) Sachs, Julian P. ; Ladd, Nemiah
    Isotopes from B. gymnorhiza mangroves in Palau during 2013. Leaf and stem samples were collected from three replicate trees on the north shore of each lake and stored frozen prior to analysis. Water samples were collected from 1cm below the lake surface, and salinity was measured with a refractometer. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/709224
  • Thesis
    Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1997-02) Sachs, Julian P.
    The goals of this thesis were: (1) to establish methods for the determination of nitrogen and carbon isotope ratios in marine particulate and sedimentary chlorophyll derivatives; (2) to establish chlorophyll δ15N and δl3C as proxies for the nitrogen and carbon isotopic composition of marine phytoplankton; and (3) to use chlorophyll nitrogen isotopic ratios to understand the origin of Late Quaternary Eastern Mediterranean sapropels. Techniques are presented for the determination of chlorin nitrogen and carbon isotopic ratios in marine particles and sediments with a precision greater than 0.15 per mil for both isotopes. The procedure can be performed in about 4 hours for particulate and 8 hours for sediment samples, and relies on multiple chromatographic purifications. About 20 g of a moderately organic-rich sediment are required. A technique is also presented for the determination of chlorin nitrogen and carbon isotopic ratios by isotope-ratio monitoring gas chromatography-mass spectrometry (irmGC-MS) by synthesizing bis-(tert.-butyldimethylsiloxy)Si(IV) chlorin derivatives. However, yields for the 4-step synthesis were only about 5-6% and there was a net isotopic depletion of 1.2 (± 0.3) per mil in the derivative, relative to the starting material. These techniques are then used to show that the nitrogen isotopic difference between chlorophyll and whole cells in six species of marine phytoplankton is 5.16 ± 2.40 per mil. For carbon, the isotopic difference between chlorophyll and whole cells in five species of marine phytoplankton is -0.02 ± 2.12 per mil. A model of the distribution of 15N in phytoplankton is constructed and it is demonstrated that the interspecies variability observed for the nitrogen isotopic difference between chlorophyll and whole cells can be attributed to differences in the partitioning of cellular nitrogen between non-protein biochemicals. In the field, where mixed assemblages of phytoplankton prevail, the isotopic difference beween chlorophyll and whole cells is expected to tend toward the average value of 5.16 per mil. Finally, the average nitrogen isotopic composition of chlorins from six Late Quaternary Eastern Mediterranean sapropels (-5.01 + 0.38 per mil) was found to be very similar to the δ15N of chlorophyll from the modem deep chlorophyll maximum (-6.38 ± 1.80 per mil) in the Eastern Mediterranean. In addition, sapropel photoautotrophic material, calculated from the chlorin δ15N, had the same isotopic composition (0.15 per mil) as both bulk sapropel sediments (-0.08 ± 0.53 per mil) and deep water nitrate (-0.05 per mil). These data suggest (a) that bottom waters were anoxic, (b) that organic matter burial efficiency was enhanced, and (c) that oligotrophic conditions similar to today persisted, in the Eastern Mediterranean during sapropel deposition. These results contradict earlier interpretations of Late Quaternary bulk sedimentary δ15N in the Eastern Mediterranean. The latter concluded that the pattern of high δ15N values in intercalated marl oozes and low values in sapropels was the result of decreased nutrient utilization, and hence, increased primary production, during sapropel events. The low δ15N of deep water nitrate in the Eastern Mediterranean suggests a significant source of new nitrogen from biological N2-fixation. It is suggested that attempts to reconstruct the nitrogen isotopic composition of marine organic matter in the past by measuring the δ15N of whole sediments may be subject to misinterpretation due to the alteration of isotopic ratios during diagenesis. The partial oxidation of marine organic matter can result in significant isotopic enrichment of the preserved residual. The magnitude of this enrichment appears to be large when bottom waters are well-oxygenated, and small when bottom waters are anoxic. Environments where large temporal reqox changes have occurred are expected to be the most problematic for the interpretation of bulk sedimentary δ15N. In these environments, the diagenetic signal can be at least as large as the primary isotopic signal being sought. The Eastern Mediterranean Sea during the Late Quaternary appears to be one such environment.
  • Dataset
    Record of abundance and δ2H of dinosterol in down core lake sediments from T Lake, Palau collected in September 2013
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2019-06-24) Sachs, Julian P. ; Dawson, Michael N.
    Record of abundance and δ2H of dinosterol in down core lake sediments from T Lake, Palau collected in September 2013 using a 5cm‐diameter Colinvaux‐Vohnout Livingstone‐type rod‐operated piston corer. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/771344
  • Dataset
    Core Logger Physical Properties for Palau Lakes Sediment Cores collected from small boats from September to October 2013
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2019-07-12) Sachs, Julian P. ; Dawson, Michael N
    Core Logger Physical Properties for Palau Lakes Sediment Cores collected from small boats from September to October 2013. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/771957
  • Article
    The 8200 year B.P. event in the slope water system, western subpolar North Atlantic
    (American Geophysical Union, 2005-04-15) Keigwin, Lloyd D. ; Sachs, Julian P. ; Rosenthal, Yair ; Boyle, Edward A.
    Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that region over the past ∼11 kyr. We focus on cores from the Laurentian Fan, which is known to have rapid and continuous accumulation of hemipelagic sediment. Although results among our various proxy data are not always in agreement, the weight of the evidence (alkenone sea surface temperature (SST), δ18O and abundance of Globigerinoides ruber) indicates a continual cooling of surface waters over Laurentian Fan, from about 18°C in the early Holocene to about 8°C today. Alternatively, Mg/Ca data on planktonic foraminifera indicate no systematic change in Holocene SST. The inferred long-term decrease in SST was probably driven by decreasing seasonality of Northern Hemisphere insolation. Two series of proxy data show the gradual cooling was interrupted by a two-step cold pulse that began 8500 years ago, and lasted about 700 years. Although this event is associated with the final deglaciation of Hudson Bay, there is no δ18O minimum anywhere in the Labrador Sea, yet there is some evidence for it as far south as Cape Hatteras. Finally, although the 8200 year B.P. event has been implicated in decreasing North Atlantic ventilation, and hence widespread temperature depression on land and at sea, we find inconsistent evidence for a change at that time in deep ocean nutrient content at ∼4 km water depth.