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Sediments from four Ocean Drilling Program sites in the Middle Valley Hydrothermal area were analyzed for thermal maturity by solid-state 1 C nuclear magnetic resonance ( C NMR) and thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR). General trends matched those found from vitrinite reflectance (Ro) profiles indicating increases in thermal maturity from 200 to 400 m in Hole 857C and from 0 to 300 m in Hole 858A. Holes 858B, 858D, and 858C all showed high maturities in samples shallower than 50 mbsf via both 1 C NMR and TG-FTIR. Some samples showed relatively high maturities via these two techniques but relatively low Ro values, suggesting that thermal alteration is reflected more rapidly via 1 C NMR and TG-FTIR than by vitrinite reflectance in these hydrothermal systems. The TG-FTIR analyses showed strong nitrogen depletion and alteration of the nitrogen-containing material in samples exposed to higher temperatures. For Hole 857C, maximum temperature exposures estimated from the vitrinite reflectance data using Ro/maximum temperature calibrations of Barker (1983) and Barker and Pawlewicz (1986) are less than 30°C at depths shallower than 47 m, 200° to 225°C at 378 m and 256° to >270°C at 433 m. These values closely match present-day temperatures of <30°C, 210° to 225°C, and 260°C, respectively, for the same intervals as measured by downhole temperature and δ 1 8 θ . Maximum temperature exposures for other Leg 139 samples for which downhole measurements were not available were estimated to be in the range of <20° to 40°C for Sample 139-855C-1R-2, 141-144 cm at 2.91 m; <20° to 70°C for Sample 139-856A-2H-4, 86-101 cm at 8.06 m; and 260° to >280°C for Sample 139-856A-13X-2,143-149 cm at 108.23 m. At the bottom of geothermally cold Hole 855C, preferential biodegradation of aliphatic portions of kerogen by aerobic microorganisms, activated by a flow of oxygenated cold seawater from basement, is postulated to be responsible for the absence of aliphatic and the predominance of aromatic carbon atoms detected by 1 C NMR. Vitrinite reflectance measurements show this section to be immature.

Time-Temperature Histories of Kerogen and Mineral Ammonia from Leg 139 (Middle Valley) Sediments