Carbon chemistry of ODP Leg 105 sites

The quantity and quality of organic carbon of Eocene to Holocene sediments from ODP Sites 645, 646, and 647 were investigated to reconstruct depositional environments. Results were based on organic-carbon and nitrogen determinations, Rock-Eval pyrolysis, and kerogen microscopy.
The sediments at Site 645 in Baffin Bay are characterized by relatively high organic-carbon values, most of which range from 0.5% to almost 3%, with maximum values in the middle Miocene. Distinct maxima of organic-carbon accumulation rates occur between 18 and 12.5 Ma and between 3.4 and 0 Ma. At Sites 646 and 647 in the Labrador Sea, organic-carbon contents vary between 0.1% and 0.75%. Cyclic 'Milankovitch-type' changes in organic-carbon deposition imply climate-controlled mechanisms that cause these fluctuations.
The composition of organic matter at Site 645 is dominated by terrigenous components throughout the entire sediment sequence. An increased content of marine organic carbon was recorded only in the late-middle Miocene. At Sites 646 and 647, the origin of the organic matter most probably is marine.
Oceanic paleoproductivity values were estimated, based on the amount of marine organic carbon. During most of the Neogene time interval at Site 645, productivity was low, i.e., similar or less than that measured in Baffin Bay today. Higher values of up to 150 (200) gC/m**2/y may have occurred only in the Miocene. At Sites 646 and 647, mean paleoproductivity values vary between 90 and 170 gC/m**2/y; i.e., these are also similar to those measured in the Labrador Sea today. Lower values of 40 to 70 gC/m**2/y were estimated for the early Eocene and (middle) Miocene.

本研究针对大洋钻探计划（Ocean Drilling Program, ODP）645、646与647站位的始新世至全新世沉积物中的有机碳含量与质量展开调查，以重建其沉积环境。分析基于有机碳与氮元素测定、Rock-Eval热解（Rock-Eval pyrolysis）以及干酪根显微分析（kerogen microscopy）。
巴芬湾645站位沉积物的有机碳含量相对较高，多数介于0.5%至近3%之间，其中中新世中期达到最高值。有机碳堆积速率的显著峰值分别出现在18~12.5 Ma以及3.4~0 Ma时段。拉布拉多海646与647站位的有机碳含量则介于0.1%至0.75%之间。沉积物有机碳沉积呈现显著的‘米兰科维奇型（Milankovitch-type）’周期性变化，暗示其波动受气候调控机制驱动。
645站位的有机质组成在整个沉积序列中均以陆源组分（terrigenous components）为主，仅在中新世中晚期记录到海洋有机碳含量的升高。而646与647站位的有机质来源大概率为海洋源。
研究基于海洋有机碳含量估算了大洋古生产力值。645站位在新近纪多数时段的古生产力水平较低，与现今巴芬湾的测量值相近或更低；最高可达150（200）gC/m²/年的高值仅可能出现在中新世时期。646与647站位的平均古生产力值介于90~170 gC/m²/年之间，与现今拉布拉多海的测量值相近；而在始新世早期及（中新世中期）时段，古生产力估算值较低，为40~70 gC/m²/年。