Short-lived thorium isotopes (234Th, 228Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean
Short-lived thorium isotopes (234Th, 228Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean
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DOI：
10.1016/s0967-0645(00)00075-8
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发表时间：
2000-01-01
影响因子：
3
通讯作者：
Fleer, A
中科院分区：
文献类型：
--
作者：
Cochran, JK;Buesseler, KO;Fleer, A
在罗斯海的三个测量站（Orca, Minke, Sei）对Th-234（溶解的，1-70和bbb70）的深度剖面进行了重复测量，用于估计从透光带输出的Th和颗粒有机碳（POC）。在1996年10月（南方早春）至1997年4月（南方秋天）期间的三次JGOFS巡航中进行了抽样。在早春巡航期间，Th-234相对于其母体U-238在100米以上的缺陷很小，在夏季增加到最大值，在秋季过程中减少。非稳态模型对Th-234数据的应用表明，来自光区的Th通量主要发生在夏季巡航和夏秋之间。罗斯海西南部的Minke站似乎比Orca或Sei站更能维持Th-234的大量移除。颗粒Th-234活度和POC在1-70粒径组分中较大，但在夏季巡航后期，当bb0 -70粒径组分的POC组分超过1-70粒径组分时除外。在bbb70 mum馏分中，POC/Th-234的比值超过了1-70 mum馏分，部分原因可能是后者的表面位置对Th的吸附更有效。颗粒Th-234通量通过乘以bbb70 mum分数（假设代表下沉颗粒）的POC/Th-234比率转换为POC通量。在1月下旬至2月上旬，POC清除模型计算的通量范围为7 ~ 91 mmol C m(-2) d(-1)，其中民科站在巡航后期观测到的通量最大。非稳态模型估计的通量在Minke（1/13-2/1/97）为85 mmol C m(-2) d(-1)，在Orca（1/19-2/1/97）为50 mmol C m(-2) d（-1）。在南部夏季巡航期间，POC库存（0-100米）的下降在罗斯海南部最为迅速（Smith et al., 2000）。南极罗斯海浮游植物生物量和初级生产力的季节循环。海洋科学进展（英文版）；Gardner et al., 2000。南极罗斯海水柱颗粒有机碳和通量的季节模式。深海研究II 47, 3423-3449)和Th-234衍生的POC通量表明，POC的下沉通量类似于POC减少的30-50%，这取决于是否使用稳态或非稳态Th通量。通过将同一样品上的Th-234和Th-228数据耦合，计算了Orca站粒子POC聚集和分解速率常数。在早春巡航后期和夏季巡航期间，POC聚集率在近地表水域最高，并随着深度的增加而降低。在同一时期，POC分解速率通常增加到最大值，在深度（> ~ 200m）处较低。在夏末，地下聚集速率增加到较高值，而分解速率降低。这一趋势有助于解释在夏季巡航后期，相对于1-70 m部分，bb0 -70 m部分的POC值更高。增加100 m以下的分解速率，使POC从大粒径组分向小粒径组分转移，并可能减弱POC下沉到透光区外的强度。(C) 2000 Elsevier Science Ltd.版权所有。
Repeated measurements of depth profiles of Th-234 (dissolved, 1-70 and > 70 mum particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). Deficiencies of Th-234 relative to its parent U-238 in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the Th-234 data shows that the flux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain significant Th-234 removal for a longer period than is evident at Orca or Sei. Particulate Th-234 activities and POC are greater in the 1-70 mum size fraction, except late in the summer cruise, when the > 70 mum POC fraction exceeds that of the 1-70 mum fraction. The POC/Th-234 ratio in the > 70 mum fraction exceeds that in the 1-70 mum fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate Th-234 fluxes are converted to POC fluxes by multiplying by the POC/Th-234 ratio of the > 70 mum fraction (assumed to be representative of sinking particles). POC fluxes calculated from a steady-state Th scavenging model range from 7 to 91 mmol C m(-2) d(-1) during late January-early February, with the greatest flux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85 mmol C m(-2) d(-1) at Minke (1/13-2/1/97) and 50 mmol C m(-2) d(-1) at Orca (1/19-2/1/97). The decline in POC inventories (0-100 m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research 47, 3119-3140; Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423-3449), and the Th-234-derived POC fluxes indicate that the sinking flux of POC is similar to 30-50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate Th-234 data with Th-228 data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (> 200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the > 70 m fraction relative to the 1-70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the Bur of POC sinking out of the euphotic zone. (C) 2000 Elsevier Science Ltd. Ail rights reserved.