Sediment Trap Time Series of GDGT and alkenone flux in the Gulf of Mexico
The tetraether index of C86 (TEX86) and alkenone unsaturation index (Uk37Õ) molecular biomarker proxies have been broadly applied in down-core marine sediments to reconstruct past sea surface temperature (SST). Although both TEX86 and Uk37 have been interpreted as proxies for mean annual SST throughout the global ocean, regional studies of glycerol dibiphytanyl glycerol tetraethers (GDGT)s and alkenones in sinking particulate matter (SPM) are required to understand the influence of seasonality, depth distribution and diagenesis on downcore variability. USGS scientists measured GDGT and alkenone flux, as well as the TEX86 and Uk37Õ indices in a 4-year sediment trap time series (2010-2014) in the northern Gulf of Mexico (nGoM), with weekly-to-monthly resolution, and compared these data with core-top sediments at the same location. GDGT and alkenone fluxes do not show a consistent seasonal cycle; however, the largest flux peaks for both occurs in winter. Uk37 co-varies with SST over the 4-year sampling interval, but the U-SST relationship in this dataset implies a smaller slope or non-linearity at high temperatures when compared with existing calibrations. Furthermore, the flux-weighted Uk37 value from sinking particles is significantly lower than that of underlying core-top sediments, suggesting preferential diagenetic loss of the tri-unsaturated alkenone in sediments. TEX86 does not co-vary with SST, suggesting production in the subsurface ocean. The flux-weighted mean TEX86 matches that of core-top sediments, suggesting that sedimentary TEX86 in the Gulf of Mexico reflects local autochthonous production. We explore these potential sources of uncertainty in both proxies in the GoM, but demonstrate that they show nearly identical trends in 20th century SST, despite these factors.
These files contain a weekly to bi-weekly resolution 4-year time series (2010-2014) of GDGT and alkenone flux in the northern Gulf of Mexico. The TEX86 and U37K' indices are also included, which are sea surface temperature proxies based on the distribution of GDGTs and alkenones, respectively.
The detailed attribute descriptions for the 2010_2014_NGOM_Sediment_Trap_Biomarkers workbook are provided in the included data dictionary (Data Dictionary). This metadata file is not complete without that worksheet.
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The McLane PARFLUX Mark 78 automated sediment trap is moored at 700 meters water depth in the northern Gulf of Mexico (27.5 ¼N and 90.3 ¼W) in 1,150 meters water depth. The trap is equipped with 21 collection cups, all mounted on a rotating plate programmed to rotate every 7 to 14 days. Sample cups were prefilled with a buffered formalin solution made with filtered seawater, formalin (3.7%) and sodium borate. The trap was recovered and redeployed every 6-9 months, a sampling gap occurred between early February 2012 and late March 2012. Sediment-trap samples were wet-split into four aliquots using a precision rotary splitter at the University of South Carolina, stored in buffered, deionized water, and then refrigerated.
Sediment trap samples were filtered onto 0.45 micron pre-combusted glass fiber filters, freeze dried, then extracted using a Dionex Accelerated Solvent Extractor (ASE 200) in 9:1 methylene chloride (DCM) to Methanol (MeOH). The total lipid extract was separated into acid and neutral fraction using aminopropyl gel columns, with neutrals eluting in 3:1 DCM:IPA and Acids eluting in 4% Acetic Acid in DCM. The neutral fraction was then separated into hydrocarbon, ketone and polar fractions via silica gel column chromatography using the elution scheme: hexane (hydrocarbons), DCM (alkenones), and MeOH (GDGTs).
The polar fraction, containing the GDGTs, was dissolved in a 99:1 (vol:vol) mixture of hexane:isopropanol, then filtered through 0.45 micron polytetrafluoroethylene (PTFE), filters. Analyses of GDGTs for TEX86 and branched and isoprenoid tetraether (BIT) index determination were performed by high?pressure liquid chromatography?mass spectrometry (HPLC?MS) at Woods Hole Oceanographic Institution. Samples were analyzed on an Agilent 1260 HPLC coupled to an Agilent 6120 MSD according to the method of Schouten et al. (2007). Briefly, A Prevail Cyano column (150 x 2.1 mm, 3 micron) was used with 100% hexane (A) and 90:10 hexane:isopropanol (v:v) (B) as eluents. Samples were injected then eluted isocratically for the first 5 minutes (min) with A, after which the eluent increased by a linear gradient up to 18% B over the next 35 min at a flow rate of 0.2 mL/min. Detection was performed in single ion monitoring mode (SIM). Synthetic C46 GDGT obtained from Purdue University was used as an internal quantification standard. All GDGT samples were analyzed in duplicate. Long-term analyses of an external laboratory standard yield a precision of 0.004 TEX86 units.
Alkenone fractions were analyzed via gas chromatography with a flame ionization detector (GC-FID) at the U.S. Geological Survey St. Petersburg Coastal and Marine Geology Center. GC-FID conditions were: split/splitless injection (inlet temperature: 300 ¼C), DB-1 capillary column (60 m, 0.32 mm i.d., 0.10 ?m film thickness), 1.5mL/min He carrier gas, with 1 microliter sample injection volume, and the following oven temperature program: 60 ¼C to 270 ¼C at 30 ¼C/minute, 270 ¼C to 310 ¼C at 1.2 ¼C/minute, and 310 ¼C to 325 ¼C at 10 ¼C/minute. Identification of alkenones was verified using GC-MS (Agilent 7890B gas chromatograph coupled to an Agilent 5977A mass spectrometer with the same column and method), and based on comparisons with a purified alkenone standard from cultured Emiliania huxleyi. Both hexatriacontane and heptatriacontane were added to samples as internal quantification standards for alkenones.
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