Down Core Oxygen Isotopic Measurements Of Diatom δ18O From The Guaymas Basin, Gulf Of California

Abstract:

The Guaymas Basin (GB), Gulf of California (27º53’N, 111º40’W ), is an evaporative basin, with sea surface temperatures (SST) varying between ~30^oC (summer) and ~15^oC (winter). Productivity is controlled mostly by seasonal upwelling starting in fall (early November) and extending into spring. We are currently analyzing δ¹⁸O_diatom from a boxcore (BC-43) using microfluorination (Menicucci, et al. 2013). This boxcore was previously analyzed for UK '37 alkenones and ²¹⁰Pb activity (Goni, et al. 2001). Residual BC-43 material was sampled at ~2cm intervals. Samples were cleaned to isolate diatoms from other sediments, then equilibrated in water with δ¹⁸O_water = +85‰ for 70 hours at 21^oC prior to vacuum dehydroxylation and microfluorination. The latter equilibration was done to account for fractionation between covalently bound O and OH^- groups during vacuum dehydroxylation, preserving the original δ¹⁸O_diatom value.

We present δ¹⁸O_diatom data from BC-43 samples covering 27cm, equivalent to >225 years of sediment accumulation. δ¹⁸O data are converted to temperature (T) based on an existing calibration (Leclerc and Labeyrie 1987). Our data suggest δ¹⁸O_diatom values record a T range of 22-18^oC, corresponding to the mixed layer depth and the chlorophyll maximum during the fall bloom. These T values are offset from SST data by a mean of 5^oC for the same sample intervals. However, δ¹⁸O_diatom values from the most recent samples suggest a ~2^oC increase in diatom T relative to SST during the last 35 years. This subsurface warming may be due to decreased fall upwelling, increased mixed layer and chlorophyll maximum depths, and/or the timing of the peak diatom bloom. Such correlations are being investigated and the latest results will be presented.

Goni, M. A., et al. (2001). Oceanographic considerations for the application of the alkenone-based paleotemperature U-37(K ') index in the Gulf of California. Geochimica Et Cosmochimica Acta 65: 545-557.

Leclerc, A. J. and L. Labeyrie (1987). Temperature dependence of the oxygen isotopic fractionation between diatom silica and water. Earth and Planetary Science Letters 84: 69-74.

Menicucci, A. J., et al. (2013). Oxygen isotope analyses of biogenic opal and quartz using a novel microfluorination technique. Rapid Communications in Mass Spectrometry 27: 1873-1881.