CPI, δ¹³C, Corg and diploptene from ODP Hole 169-1034B varved sediments@en

Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1-2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17alpha(H),21beta(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ~6000 years BP up to present time to about 70 µg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (<51 µg/g Corg) and even slightly decrease to 36 µg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ~6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between ~31.5 and ~39.6 per mil PDB after ~6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic-anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate delta13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at ~31.4 ± 1.1 per mil PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ~6000 years BP, diploptene enriched in 13C of up to -26.3 per mil PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.萨尼奇湾（Saanich Inlet）自末次冰期以来便是一处生产力极高的峡湾。在大洋钻探计划（Ocean Drilling Program, ODP）第169S航次中，研究人员于该湾中部的1034号钻孔钻取获得了近70米厚的全新世沉积物。其中，浅层年轻沉积物呈层纹状、处于厌氧环境且富含有机质（有机碳（Corg）含量为1~2.5 重量百分比（wt.%））；而70米海底以下（meters below seafloor, mbsf）的较老沉积物则无层理构造、处于有氧环境，具有冰海相沉积特征，且有机质含量显著更低。这种沉积与环境差异同样体现在孔隙流体组成、生物标志物谱及其碳同位素信号的变化中。 源自细菌的藿烷类化合物17α(H),21β(H)-藿-22(29)-烯（双藿烯，diploptene）在萨尼奇湾全新世沉积物中广泛分布，但在更新世冰海相沉积物中未被检出。该化合物的浓度在距今约6000年（Before Present, BP）之后持续升高，至现今达到约70 µg/g Corg；而陆源生物标志物如正构烷烃C31在整个全新世期间均处于较低水平（<51 µg/g Corg），甚至在最近时期略微下降至36 µg/g Corg。 距今约6000年以来的沉积物中双藿烯浓度的显著升高，将近期一段更高初级生产力、底层水缺氧程度更强、细菌活性更高的时期，与此前未被明确区分的活性较低的较早时期清晰区分开来。全新世双藿烯的碳同位素组成在距今约6000年之后介于~31.5‰至~39.6‰ PDB（佩德贝莱姆尼特标准，Peedee Belemnite, PDB）之间。这类碳同位素记录差异可能反映了生活在上覆水体氧跃层的细菌群落结构变化：较重的同位素值与硝化细菌的活动特征相符，较轻的同位素值则对应有氧甲烷氧化细菌的活动，因此中等δ13C值大概率代表这两类菌群的混合信号。 与之形成鲜明对比的是，正构烷烃C31的碳同位素组成在整个全新世期间大致稳定在~31.4 ±1.1‰ PDB，表明其始终来源于高等植物的角质层蜡质。在距今约6000年之前，富集13C的双藿烯（δ13C可达-26.3‰ PDB）指示了透光带中蓝细菌的存在，同时也表明全新世早期存在一段初级生产力较低、底层水含氧程度更高、因而细菌活性更低的时期。