GEOTHERMAL REGIME OF THE WESTERN MARGIN OF THE GREAT BAHAMA BANK
GEOTHERMAL REGIME OF THE WESTERN MARGIN OF THE GREAT BAHAMA BANK
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大巴哈马浅滩西缘的地热状况
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发表时间：
2000
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通讯作者：
Kelin Wang
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作者：
S. Nagihara;Kelin Wang
利用大洋钻探计划第166航次期间和之后获得的井底温度和热导率测量结果，对大巴哈马浅滩西部边缘的地热状况进行了检查。本研究的重点是从1003至1007站点的钻孔样带的数据。这些数据揭示了两个重要的观测特征。首先，所有钻井地点的温度与累积热阻曲线在40至100 mbsf的深度范围内显示出显著的曲率。他们往往是凹向上的形状。第二，传导背景热流值为这五个钻井网站，确定从深，线性部分的地热剖面，显示出系统的变化沿着钻井样带。在远离河岸的海底，热流为43-45 mW/m2，并在上坡处降低至~35 mW/m2。我们研究三种机制作为弯曲的地热剖面的潜在原因。它们是：（1）最近沉积速率的增加，（2）海水流入浅层沉积物，（3）底层水温（BWT）的时间波动。我们的分析表明，第一种机制是可以忽略的。第二种机制可以解释来自站点1004和1005的数据。第三种机制最容易解释1006位点的温度分布。我们重建的历史BWT在这个网站上通过解决逆热传导问题。反演结果表明，整个世纪气候变暖约1°C，与西亚热带大西洋的水文和气候资料一致。然而，1003和1007站点的数据似乎没有显示出这种趋势。因此，这里测试的三种机制都不能解释所有钻井现场的观察结果。至于背景热流沿着钻孔断面的横向变化，我们认为很大程度上是由地形变化的热效应引起的。我们通过获得一个二维的解析解来模拟这种效果。该模型表明，该地区的背景热流为~43 mW/m2，该值类似于在佛罗里达碳酸盐台地对面的墨西哥湾确定的背景热流。在大洋钻探计划（ODP）第166航次之前，佛罗里达-巴哈马近海碳酸盐岩平台的直接地热测量很少。使用传统的海洋热流探针进行测量的报道还没有，如Bullard（1954）、Jemsek等人（1985）和Lister等人（1990）所描述的。这可能是因为在浅水环境中确定地热热流的困难。佛罗里达海峡的大部分海底都浅于海平面以下800米。底层海水的温度存在显著的季节性波动（例如，Niiler和Richardson，1973年）。通常情况下，地热探测器只能穿透海底以下5-7米，而与季节性波动相关的热噪声可能穿透深度超过10 mbsf。据报道，在佛罗里达西南部的海上进行了一些钻孔温度测量（Buffler等人，1984年），但巴哈马平台实际上是“未受影响的”。来自166航段的现场井底温度数据是在30-300 mbsf的深度获得的，可以提供关于平台热状况的第一直接信息。在166航段期间获得的现场井底温度和热导率在1003至1009站点共进行了62次可靠的现场井底温度测量（图1）。两种类型的仪器用于测量：Adara temper 1 Swart，P.K.，Eberli，G. P.，马龙，M. J.，和Sarg，J.F.（编），2000. Proc. ODP，Sci.结果，166：德克萨斯州学院站（大洋钻探计划）。2休斯顿大学地球科学系，4800 Calhoun，Houston TX 77204-5503，USA。nagihara@uh.edu 3加拿大地质调查局太平洋地球科学中心，9860 West Saanich Road，Sidney，BC V8 L 4 B2，Canada。温度探头（WSTP），其水取样器关闭。Eberli、Swart、马龙等人（1997年）和先前的ODP出版物（如Fisher和Becker（1993年））中描述了这些仪器。Adara工具记录温度，同时APC保持在200 20 0 200的底部。
The geothermal regime of the western margin of the Great Bahama Bank was examined using the bottom hole temperature and thermal conductivity measurements obtained during and after Ocean Drilling Program (ODP) Leg 166. This study focuses on the data from the drilling transect of Sites 1003 through 1007. These data reveal two important observational characteristics. First, temperature vs. cumulative thermal resistance profiles from all the drill sites show significant curvature in the depth range of 40 to 100 mbsf. They tend to be of concave-upward shape. Second, the conductive background heat-flow values for these five drill sites, determined from deep, linear parts of the geothermal profiles, show a systematic variation along the drilling transect. Heat flow is 43–45 mW/m2 on the seafloor away from the bank and decreases upslope to ~35 mW/m2. We examine three mechanisms as potential causes for the curved geothermal profiles. They are: (1) a recent increase in sedimentation rate, (2) influx of seawater into shallow sediments, and (3) temporal fluctuation of the bottom water temperature (BWT). Our analysis shows that the first mechanism is negligible. The second mechanism may explain the data from Sites 1004 and 1005. The temperature profile of Site 1006 is most easily explained by the third mechanism. We reconstruct the history of BWT at this site by solving the inverse heat conduction problem. The inversion result indicates gradual warming throughout this century by ~1°C and is agreeable to other hydrographic and climatic data from the western subtropic Atlantic. However, data from Sites 1003 and 1007 do not seem to show such trends. Therefore, none of the three mechanisms tested here explain the observations from all the drill sites. As for the lateral variation of the background heat flow along the drill transect, we believe that much of it is caused by the thermal effect of the topographic variation. We model this effect by obtaining a two-dimensional analytical solution. The model suggests that the background heat flow of this area is ~43 mW/m2, a value similar to the background heat flow determined for the Gulf of Mexico in the opposite side of the Florida carbonate platform. INTRODUCTION Before Ocean Drilling Program (ODP) Leg 166, there were very few direct geothermal measurements in offshore Florida-Bahama carbonate platforms. There has been no report of measurements using conventional marine heat-flow probes such as those described by Bullard (1954), Jemsek et al. (1985), and Lister et al. (1990). This is probably because of the difficulty associated with determining geothermal heat flow in a shallow-water environment. Much of the seafloor in the Straits of Florida is shallower than 800 meters below sea level (mbsl). There is significant seasonal fluctuation in the temperature of the bottom seawater (e.g., Niiler and Richardson, 1973). Normally, a geothermal probe penetrates only 5–7 meters below seafloor (mbsf), while the thermal noise associated with the seasonal fluctuation may penetrate deeper than 10 mbsf. Some borehole temperature measurements were reported offshore of southwestern Florida (Buffler et al., 1984), but the Bahama platform was virtually “untouched.” The in situ bottom-hole temperature data from Leg 166, which were obtained in depths of 30–300 mbsf, may provide the first direct information on the thermal regime of the platform. IN-SITU BOTTOM-HOLE TEMPERATURES AND THERMAL CONDUCTIVITIES OBTAINED DURING LEG 166 A total of 62 reliable in situ bottom-hole temperature measurements were made at Sites 1003 through 1009 (Fig. 1). Two types of instrumentation were used for the measurements: the Adara temper1Swart, P.K., Eberli, G.P., Malone, M.J., and Sarg, J.F. (Eds.), 2000. Proc. ODP, Sci. Results, 166: College Station TX (Ocean Drilling Program). 2Department of Geosciences, University of Houston, 4800 Calhoun, Houston TX 77204-5503, USA. nagihara@uh.edu 3Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, B.C. V8L 4B2, Canada. ature tool, which was built into the cutting shoe of the advanced piston corer (APC), and the water-sampling temperature probe (WSTP) with its water sampler turned off. These instrumentations have been described in Eberli, Swart, Malone, et al. (1997) and previous ODP publications such as Fisher and Becker (1993). The Adara tool records the temperature while the APC is held at the bottom of the 200 20 0 200