Collaborative Proposal: Using Radiocarbon Measurements of Benthic Megafauna as a Tool for Assessing Bentho-Pelagic Coupling in the Marine Organic Carbon Cycle

该项目研究了沉积在大陆边缘的有机碳的性质的季节性变化。到达海底的有机碳通量可以通过使用颗粒捕捉器或通过模拟海底埋藏和再生速率来量化，但由于与来自海洋和陆地系统的旧难熔碳共沉积，在大陆边缘很难评估有机碳的质量(即其稳定性)。炸弹碳-14(C14)是最近产生的海洋有机物的有用示踪剂，但在大陆边缘颗粒圈闭样品和表层沉积物中，这一信号被较老的难熔碳掩盖了，这些碳从较浅的海洋沉积物中重新悬浮和重新加工。已有证据表明，大陆边缘系统中的表层沉积物补给器由于其选择性的吸收和消化过程，能够隔离到达海底的总有机通量中的不稳定部分。因此，这些底栖动物的身体组织和肠道沉积物可以提供到达大陆边缘海底的不稳定物质的记录。最初的C14数据表明，与它们生活和觅食的表层沉积物相比，表层沉积喂食者的身体组织中这种放射性同位素的丰度为每毫升100至300毫升。根据质量平衡C14计算，不稳定组分可能只占表层海床总有机质的十分之几，而表层沉积物捕食者在其身体组织中(每毫升+20到+80)表现出C14特征，主要是当地炸弹产生的信号。来自大陆边缘的钍(Th)数据表明，最近在大陆边缘海底沉积的有机质中，多达50%可能在微生物降解或埋葬之前通过沉积物喂食者的肠道。因此，对底栖动物的C14测量可以为评估有机物到达海底的不稳定性和追踪这种物质在海底食物网中的命运提供独特的工具。许多现有的成岩模型将微生物和沉积物供给者的有机质再生视为一个批处理过程，对大部分有机质只有一个碳降解常数。C14数据清楚地表明，摄取有机物和消化这种物质是非常有选择性的过程。来自加州边界和南极大陆边缘的C14分析表明，这种方法在记录表层沉积物有机碳不稳定性的变化方面是有用的。德马斯特和史密斯将扩大初步研究，以评估沉积在海床上的有机碳质量的季节性变化。在6次前往加州边界和5次前往南极大陆边缘的航行中，采集了颗粒捕捉器样本、底栖动物组织、肠道样本和表层沉积物，涵盖了有机碳供应的主要季节变化。需要资金对这些样品进行150次C14分析(以及补充的C13、N15和C/N测量)，以便能够确定量化到达海底的有机物质的性质和通量的时间序列。他们不仅将评估颗粒选择和有机物消化选择过程的性质的季节变化，而且还将评估由于海底的进食策略(海底表层沉积物喂食者与地下沉积物喂食者与头朝下的地下沉积物喂食者)而导致的这些过程的变化。

This project examines the seasonal variation in the nature of the organic carbon deposited on continental margins. The flux of organic carbon reaching the seabed can be quantified by using particle traps or by modeling seabed burial and regeneration rates, but the quality (i.e., its lability) has been difficult to assess on continental margins because of co-deposition with old refractory carbon from marine and terrestrial systems. Bomb Carbon-14 (C14) is a useful tracer of recently produced marine organic matter, but this signal is masked in continental margin particle?trap samples and surface sediments by the older and refractory carbon that is re-suspended and reworked from shallower marine deposits. Surface deposit feeders in continental margin systems have been shown to sequester the labile fraction of the total organic flux reaching the seabed as a result of their selective ingestive and digestive processes. Therefore, the body tissues and the gut sediments of these benthic fauna can provide a record of the labile material reaching the continental margin seafloor. Initial C14 data indicate that the body tissues of surface depositing feeders are enriched in this radioisotope by 100 to 300 per mil relative to the surface sediments in which they live and feed. Based on mass?balance C14 calculations, the labile component may comprise only a few tenths of a percent of the total organic matter in the surface seabed, yet the surface deposit feeders exhibit C14 signatures in their body tissues (+20 to +80 per mil) dominated by the local bomb?produced signal . Thorium (Th) data from continental margins suggest that as much as 50% of the recently deposited organic matter on the continental margin sea floor may pass through the guts of deposit feeders prior to microbial degradation or burial. Therefore, C14 measurements on benthic fauna can provide a unique tool for assessing the lability of organic matter reaching the seafloor and for tracking the fate of this material in the benthic food web. Many existing diagenetic models treat organic matter regeneration by microbes and deposit feeders as a batch process with a single carbon degradation constant for the bulk organic matter. The C14 data clearly show that ingestion of organic matter and digestion of this material are very selective processes. The C14 analyses from the California Borderland and the Antarctic continental margin have shown the utility of this approach for documenting variations in the organic carbon lability of surface sediments.Drs. DeMaster and Smith will expand the initial studies to enable assessment of seasonal variations in the quality of organic carbon deposited on the seabed. Particle?trap samples, benthic?faunal tissues, gut samples and surface sediments have been collected during 6 cruises to the California Borderland and 5 cruises to the Antarctic continental margin covering the major seasonal variations in organic carbon supply. Funds are requested for making 150 C14 analyses (and complementary C13, N15, and C/N measurements) on these samples so that a time series quantifying the nature and flux of organic matter reaching the seabed can be determined. They will assess not only seasonal variations in the nature of particle selection and digestive selection processes for organic matter, but also the changes in these processes as a result of feeding strategy (epibenthic surface deposit feeders, vs. subsurface deposit feeders vs. head?down subsurface deposit feeders) on the seafloor.