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中更新世以来热带西太平洋深部[CO32-]演化:对大洋碳储存及大气pCO2变化的启示
秦秉斌1,2
学位类型博士
导师李铁刚
2017-05
学位授予单位中国科学院大学
学位授予地点北京
学位专业海洋地质学
关键词热带西太平洋 浮游有孔虫 Snw [Co32-] 碳循环演化
摘要浮游有孔虫壳体重量在碳酸钙溶解作用影响下变轻,故可以作为一种古海洋代用指标来重建过去大洋深部碳酸根离子浓度([CO32-]),从而为理解过去大洋碳循环演变及其气候影响提供依据。本研究首先改进了粒径标准化壳体重量(SNW)测定清洗方案,之后建立了热带西太平洋浮游有孔虫Globigerinoides sacculiferNeogloboquadrina dutertreiPulleniatina obliquiloculata的现代SNW-碳酸根离子饱和程度(Δ[CO32-])经验公式,并评估SNW指标在热带西太平洋的适用性,然后将公式应用于该海域WP7和MD06-3047b孔以重建热带西太平洋中更新世以来深部[CO32-]变化,并探讨期间大洋碳循环演化及其对气候的影响,最后讨论P. obliquiloculata初始粒径标准化壳体重量(ISNW)的受控因素。
传统浮游有孔虫壳体重量清洗方案(壳体置于去离子水中超声清洗8 s)无法完全去除有孔虫壳体房室内杂质,且过长的超声清洗时间导致样品中重量较轻的壳体优先破碎,这一过程使最终结果偏重。本研究发现六偏磷酸钠溶液的超声清洗效率远好于去离子水,将实验清洗方案改进为将壳体置于2 %六偏磷酸钠溶液中超声清洗4 s,该清洗方案不仅提高壳体清洗效果,而且能有效避免壳体在超声清洗中破碎,使最终壳体重量结果更加准确。
基于改进的SNW测定方案,研究根据热带西太平洋表层沉积物与柱状样顶层样品建立了浮游有孔虫G. sacculiferN. dutertreiP. obliquiloculata的SNW-Δ[CO32-]经验公式,以上公式适用于西太平洋1.6-3.9 km水深样品。
末次冰期最盛期(LGM)以来热带西太平洋WP7和KX97322-4孔浮游有孔虫SNW结果表明过去G. sacculiferN. dutertrei SNW并没有显著受到孔隙水溶解效应以及初始壳体重量变化的影响,而是主要反映深部Δ[CO32-]变化。而抗溶种P. obliquiloculataSNW还受到初始壳体重量变化的影响。
基于WP7孔G. sacculiferN. dutertrei SNW重建的250 ka以来热带西太平洋暖池核心区深部[CO32-]数据显示:LGM以来[CO32-]大体保持不变,认为可能是数种机制共同作用导致了LGM期间大气pCO2的下降;[CO32-]在MIS 5e-3和MIS 7-6期间呈现整体上升趋势,该结果支持“珊瑚礁假说”,认为太平洋深部碳酸盐系统响应全球海平面变化;Terminations 2和3期间记录显示的[CO32-]高值事件与“冰消期流通性假说”吻合,认为大洋垂直流通性的减弱使CO2从深海被排放入大气,并在大洋深部引起碳酸钙保存事件;MIS 4期间热带西太平洋深部[CO32-]下降,结合其他大洋的[CO32-]/碳酸钙溶解作用重建记录,认为MIS 4期间全球大洋深部的碳存储量增加,并引发了一次全球性的碳酸钙溶解事件。
基于MD06-3047b孔N. dutertrei SNW重建的700 ka以来热带西太平洋边缘深部[CO32-]数据显示:[CO32-]在冰期、冰消期时较高,而在间冰期时较低,是典型的太平洋型CaCO3旋回特征;在冰期-间冰期旋回尺度中,深部[CO32-]响应海平面变化;MIS 11期间,[CO32-]急剧下降至~40 μmol kg-1,该溶解事件可能是同期全球大洋颗石藻生产力激增引起;在整体趋势方面,[CO32-]记录并没有显示广泛出现在大洋δ13C记录中的400-500 kyr长周期变化,其中原因还需要进一步研究探讨。
研究利用WP7和MD06-3047b孔中P. obliquiloculata SNW以及基于N. dutertrei SNW重建的Δ[CO32-]得出P. obliquiloculata ISNW数据。P. obliquiloculata ISNW主要反映次表层水体[CO32-]变化。WP7孔ISNW结果显示:250 ka以来,P. obliquiloculata ISNW在冰期时比间冰期轻,而在冰消期显著增重,ISNW主要受到厄尔尼诺/南方涛动(ENSO)影响下温跃层变化的调控。MD06-3047b孔ISNW结果显示:700 ka以来的长时间尺度上,P. obliquiloculata ISNW可能受到夏季风强度变化控制,即夏季风增强导致当地温跃层变浅,次表层水体[CO32-]下降,并使P. obliquiloculata ISNW变轻。
其他摘要The shell weights of planktonic foraminifera, which undergoes changes with calcite dissolution, can be used as a proxy to reconstruct deep-water carbonate ion concentrations ([CO32-]) in the past and, therefore, to provide clues regarding the ocean carbon cycle and global climate influence. In this study, we provide an improved protocol for cleaning of planktonic foraminifera for size-normalized weight (SNW) measurement. Then we present new SNW-carbonate ion concentration saturation levels (Δ[CO32-]) core-top calibrations for three planktonic foraminiferal species Globigerinoides sacculifer, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata and assess their reliability as a paleo-alkalinity proxy. Based on these calibrations, we reconstruct SNW-based deep-water [CO32-] for core WP7 and MD06-3047b from the western tropical Pacific since the middle Pleistocene. Our findings provide new insights into the role of the deep Pacific in the global carbon cycle and the causes of changing atmospheric CO2. Finally, we discuss the factors controlling the initial size-normalized weight (ISNW) of P. obliquiloculata.
Foraminiferal shells must be cleaned prior to weight measurements in order to remove detritus trapped within the shell chambers. The standard cleaning procedure, which involves an eight-second sonication step in water, causes greater breakage of thin-walled shells relative to thick-walled shells, thereby biasing weight measurements. Furthermore, the extent of shell breakage is greater for samples that have undergone significant in-situ partial dissolution. Because the sonic cleaning efficiency of sodium hexametaphosphate is much greater than that of water, we recommend a cleaning procedure based on a four-second sonication step in a 2% sodium hexametaphosphate solution. This procedural change provides a greater detritus cleaning effect and minimizes shell breakage, making shell weight measurements more reliable.
New core-top SNW-Δ[CO32-] calibrations of the planktonic foraminifera G. sacculifer, N. dutertrei, and P. obliquiloculata in the western tropical Pacific were established by using our improved cleaning procedure (i.e., soaking plus a four-second sonication step in 2 % sodium hexametaphosphate solution). These calibrations are valid over the water depth range of 1.6 to 3.9 km in the western Pacific.
For the purpose of testing the reliability of SNW proxy, we determined SNWs for the planktonic foraminiferal species G. sacculifer, N. dutertrei, and P. obliquiloculata from cores KX97322-4 and WP7 since the LGM. The results indicate that variation in respiratory dissolution or initial shell weight was not a major factor controlling shell weight of G. sacculifer and N. dutertrei in the sediment record from the Pacific Ocean. However, the SNWs of P. obliquiloculata were affected by environmental factors in addition to deep-water Δ[CO32-].
We reconstruct SNW-based deep-water [CO32-] for core WP7 from the western tropical Pacific warm pool since 250 ka. Secular variation in the SNW proxy documents little change in deep Pacific [CO32-] between the Last Glacial Maximum (LGM) and the Holocene. Further back in time, deep-water[CO32-] shows long-term increases from marine isotope stage (MIS) 5e to MIS 3 and from early MIS 7 to late MIS 6, consistent with the “coral reef hypothesis” that the deep Pacific Ocean carbonate system responded to declining shelf carbonate production during these two intervals. During deglaciations, we have evidence of [CO32-] peaks coincident with Terminations 2 and 3, which suggests that a breakdown of oceanic vertical stratification drove a net transfer of CO2 from the ocean to the atmosphere, causing spikes in carbonate preservation (i.e., the “deglacial ventilation hypothesis”). During MIS 4, a transient decline in SNW-based [CO32-], along with other reported [CO32-] and/or dissolution records, implies that increased deep-ocean carbon storage resulted in a global carbonate dissolution event.
We also reconstruct SNW-based deep-water [CO32-] for core MD06-3047b from the margin of the western tropical Pacific warm pool since 700 ka. [CO32-] is higher during glacials and deglacials, and lower during interglacials. This pattern is consistent with the Pacific-style deep-sea sedimentary carbonate cycles. Deep Pacific Ocean carbonate system responded to glacial-interglacial changes in shelf carbonate production since 700 ka. In addition, [CO32-] reached minimum of ~40 μmol kg-1 during MIS 11. Increased carbonate dissolution during the Mid-Brunhes Dissolution Interval may be caused by a global increase in pelagic coccolith carbonate production. Furthermore, the long-term cyclicity of 400-500 kyr observed in δ13C records around the globel ocean is not obsearved in our [CO32-] record. Further work will be required to determine the causes for this discrepancy.
P. obliquiloculata ISNWs for core WP7 and MD06-3047b are calculated using P. obliquiloculata SNWs and N. dutertrei SNW-based deep-water Δ[CO32-]. P. obliquiloculata ISNW seems to be related mainly to subsurface-water [CO32-]. P. obliquiloculata ISNW for core WP7 is heavier during interglacials and deglacials, and lighter during glacials. This pattern of ISNW variation is related to changes of thermocline gradient associated with El Niño/LaNiña-Southern Oscillation (ENSO). However, long-term changes in P. obliquiloculata ISNW for core MD06-3047b seem to be related to changes of thermocline gradient associated with East Asian Summer Monsoon (EASM). Enhanced wind-driven circulation caused by strengthened ESAM would introduce a shoaling of the thermocline. As a result, a drawdown of subsurface-water [CO32-] would have caused the P. obliquiloculata ISNW to decrease.
学科领域海洋地质学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/136538
专题海洋地质与环境重点实验室
作者单位1.中国科学院海洋研究所
2.中国科学院大学
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GB/T 7714
秦秉斌. 中更新世以来热带西太平洋深部[CO32-]演化:对大洋碳储存及大气pCO2变化的启示[D]. 北京. 中国科学院大学,2017.
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