Glacial-interglacial record of ODP Site 181-1119 in the Canterbury Bright, Southwest Pacific Ocean

Ocean Drilling Program (ODP) Site 1119 is located at water depth 395 m near the subtropical front (STF; here represented by the Southland Front), just downslope from the shelf edge of eastern South Island, New Zealand. The upper 86.19 metres composite depth (mcd) of Site 1119 sediment was deposited at an average sedimentation rate of 34 cm/kyr during Marine Isotope Stages (MIS) 1-8 (0-252 ka), and is underlain across a ~25 kyr intra-MIS 8 unconformity by MIS 8.5-11 (277-367 ka) and older sediment deposited at ~14 cm/kyr. A time scale is assigned to Site 1119 using radiocarbon dates for the period back to ~39 ka, and, prior to then, by matching its climatic record with that of the Vostok ice core, which it closely resembles. Four palaeoceanographic proxy measures for surface water masses vary together with the sandy-muddy, glacial-interglacial (G/I) cyclicity at the site. Interglacial intervals are characterised by heavy delta13C, high colour reflectance (a proxy for carbonate content), low Q-ray (a proxy for clay content) and light delta18O; conversely, glacial intervals exhibit light delta13C, low reflectance, high Q-ray and heavy delta18O signatures. Early interglacial intervals are represented by silty clays with 10-105-cm-thick beds of sharp-based (Chondrites-burrowed), shelly, graded, fine sand. The sands are rich in foraminifera, and were deposited distant from the shoreline under the influence of longitudinal flow in relatively deep water. Glacial intervals comprise mostly micaceous silty clay, though with some thin (2-10 cm thick) sands present also at peak cold periods, and contain the cold-water scallop Zygochlamys delicatula. Interglacial sandy intervals are characterised by relatively low sedimentation rates of 5-32 cm/kyr; cold climate intervals MIS 10, 6 and 2 have successively higher sedimentation rates of 45, 69 and 140 cm/kyr. Counter-intuitively,and forced by the bathymetric control of a laterally-moving shoreline during G/I and I/G transitions, the 1119 core records a southeasterly (seaward) movement of the STF during early glacial periods, accompanied by the incursion of subtropical water (STW) above the site, and northwesterly (landward) movement during late glacial and interglacial times, resulting in a dominant influence then of subantarctic surface water (SAW). The history of passage of these different water masses at the site is clearly delineated by their characteristic delta13C values. The intervals of thin, graded sands-muds which occur within MIS 2-3, 6, 7.4 and 10 indicate the onset at times of peak cold of intermittent bottom currents caused by strengthened and expanded frontal flows along the STF, which at such times lay near Site 1119 in close proximity to seaward-encroaching subantarctic waters within the Bounty gyre. In common with other nearby Southern Hemisphere records, the cold period which represents the last glacial maximum lasted between ~23-18 ka at Site 1119, during which time the STF and Subantarctic Front (SAF) probably merged into a single intense frontal zone around the head of the adjacent Bounty Trough.

海洋钻探计划（Ocean Drilling Program, ODP）1119站位位于新西兰南岛东岸陆架外缘稍下坡处，水深395米，毗邻亚热带锋面（subtropical front, STF；本文以新西兰南兰德锋面作为其代表）。该站位1119的沉积物复合深度（composite depth, mcd）上部86.19米沉积于海洋同位素阶段（Marine Isotope Stages, MIS）1-8（0~252 ka），平均沉积速率为34 cm/kyr；其下以约25 kyr的MIS 8内部不整合面（unconformity）为界，覆盖了MIS 8.5~11（277~367 ka）及更老的沉积物，后者沉积速率约为14 cm/kyr。1119站位的年代标尺通过~39 ka以来的放射性碳定年（radiocarbon dates）结果构建，更早的时段则通过将该站位的气候记录与高度相似的沃斯托克冰芯（Vostok ice core）进行匹配来确定。该站位的四项古海洋学代用指标（palaeoceanographic proxy measures），用于表征表层水团（surface water masses），与砂泥质沉积物的冰期-间冰期（glacial-interglacial, G/I）旋回同步变化。间冰期段以δ¹³C（delta13C）偏正、高颜色反射率（colour reflectance，碳酸盐含量代用指标）、低Q射线（Q-ray，黏土含量代用指标）及δ¹⁸O（delta18O）偏负为特征；反之，冰期段则表现为δ¹³C偏负、低反射率、高Q射线及δ¹⁸O偏正的特征。早期间冰期段以粉砂质黏土（silty clays）为代表，夹有10~105厘米厚的锐底（经Chondrites潜穴生物扰动）、含壳体、具粒序层理的细砂层。这些砂层富含有孔虫（foraminifera），形成于离岸较远的相对深水环境，受纵向流作用沉积。冰期段主要由含云母粉砂质黏土（micaceous silty clay）组成，不过在冷期峰值阶段也存在少量厚度2~10厘米的砂层，并含有冷水扇贝Zygochlamys delicatula。间冰期砂质沉积段的沉积速率相对较低，为5~32 cm/kyr；而MIS 10、6和2等冷气候段的沉积速率依次升高，分别为45、69和140 cm/kyr。与直觉相悖的是，受冰期-间冰期及间冰期-冰期转换时期岸线侧向移动的水深地形调控（bathymetric control），1119岩芯记录到：早冰期阶段，亚热带锋面（STF）向东南向（向海方向）移动，伴随亚热带水团（subtropical water, STW）侵入站位上方；晚冰期及间冰期时期，STF则向西北向（向陆方向）移动，此时亚南极表层水（subantarctic surface water, SAW）占据主导地位。站位处不同水团的活动历史可通过其特征性的δ¹³C值清晰区分。出现于MIS 2~3、6、7.4和10段的薄层粒序砂-泥层，指示了峰值冷期时段间歇性底流（bottom currents）的出现——该底流由沿STF增强并扩张的锋面流（frontal flows）引发，此时STF紧邻邦迪环流（Bounty gyre）内向海推进的亚南极水域，距1119站位极近。与南半球其他邻近记录一致，1119站位记录的末次冰盛期（last glacial maximum）持续时长约为23~18 ka；此时期STF与亚南极锋（Subantarctic Front, SAF）可能合并为单一强锋带，位于邻近的邦迪海槽（Bounty Trough）头部附近。