南大洋表层海水酸化研究进展

海洋酸化是全球变化的一个关注点,南大洋CO2吸收量占海洋对人为CO2的吸收量的30%—40%,同时,南大洋是缓冲能力较低、生态系统脆弱的高纬海域,所以,理解南大洋海水酸化过程及其控制因素就显得尤为重要。海水酸化引起海水中碳酸根离子(CO2-3)的浓度、碳酸钙饱和度(Ω)降低,这对成钙类生物碳酸钙(包括文石和方解石)的形成有害,最终可能对生态系统造成危害。了解海洋酸化的影响、预测酸化对海水碳酸盐体系和生态系统的影响具有重要意义。因此,本文对目前南大洋的开阔大洋区以及两个具有代表性的边缘海和海湾的酸化研究概况进行了梳理,试图对南大洋表层海水酸化的研究进行概述、展望未来南大洋海洋酸化研究趋势并提出一些亟须解决的问题。     

珊瑚礁海水pH变化及其对海洋酸化的意义

珊瑚钙质骨骼δ11B重建的海水pH时间变化序列显示：珊瑚礁海水pH的长期变化具有显著的年际―年代际波动周期,并且这些波动是区域海洋气候过程和珊瑚礁海水生物活动共同作用的结果。珊瑚礁生态系统中的生物活动（光合呼吸作用和钙化作用）控制着海水碳酸盐系统的组成,调节着海水pH的变化。这一过程在全球气候环境变化和区域海气过程的影响下,一起作用于珊瑚礁海水pH的变化,因而使得不同海域的珊瑚礁海水对海洋酸化的响应有所差别。研究珊瑚礁生物活动在长时间尺度上对海水pH的作用对认识珊瑚礁海水酸化机理十分重要,同时也是了解珊瑚礁生态系统如何响应海洋酸化的重要手段。     

气溶胶核化清除的化学效应──Ⅱ：动力学参数对云滴化学非均匀性的影响

运用已建立的气溶胶核化清除的物理化学模式，研究了云的动力学因子（如：气块上升速度、夹卷作用）对云滴化学非均匀性的影响。计算结果表明：较强烈的云发展（较大的气块上升速度）可加强由于气溶胶核化和云滴凝结增长造成的云滴化学的非均匀程度。夹卷作用抑制了云的发展，因而减弱了这种非均匀程度。夹卷作用同时也造成总体液态水中Ｓ（ＶＩ）、Ｈ＋等浓度的增加，在Ｓｍａｘ附近可达１个量级。如果考虑气溶胶粒子的夹卷，则可使气块内云滴污染物浓度随云滴大小的变化更加复杂化，如：不仅云滴污染物浓度随云滴大小而变化，即使对于相同大小的云滴之间，其污染物浓度也可相差很大。     

东西伯利亚海海冰季节变化特征及主要影响因素分析

利用1997-2005年美国国家冰雪中心提供的卫星遥感数据,对东西伯利亚海海冰周年变化特征及其动力和热力学机制进行详细分析,以1999年海冰状况为例讨论了该海域海冰的周年变化。按照海冰变化的区域特征和融化机制差异,将全年的海冰变化过程分为密集冰封期、陆坡开裂期、西部融化期、全面融化期和秋季结冰期。不同年份各个阶段发生的具体日期不尽相同,海冰覆盖面积最小值及其发生时间有所差异,但是,各年海冰变化的5个阶段都清晰可辨。海冰融化时间持续3个月,冻结时间仅为1个半月左右。每年5月份东西伯利亚海陆坡处海冰发生开裂,主要是该时期风场辐散的作用。1999年,除5月份以外的其他月份,东西伯利亚海海表面风场是辐聚风场,不利于海冰融化和开阔水域面积的扩大。东西伯利亚海海冰融化的决定性因素是陆地径流,因迪吉尔卡河、科雷马河、亚纳河和勒拿河四条河流在海冰融化过程中发挥主要作用。海冰覆盖面积最小值出现的时间一般是9月下旬,整个海域的沿岸带海冰全部消失,形成大范围的开阔水。夏季北半球气温的升高和太阳辐射的加热作用,为海冰融化提供持续的热量。     

Nonsulfide and sulfide-rich zinc mineralizations in the Vazante, Ambrósia and Fagundes deposits, Minas Gerais, Brazil: Mass balance and stable isotope characteristics of the hydrothermal alterations

The Vazante Group hosts the Vazante nonsulfide zinc deposit, which comprises high-grade zinc silicate ore (ZnSiO₄), and late-diagenetic to epigenetic carbonate-hosted sulfide-rich zinc deposits (e.g. Morro Agudo, Fagundes, and Ambrósia). In the sulfide-rich deposits, hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration (e.g. Fagundes) or with direct interaction with the metalliferous fluid (e.g. Ambrósia). At Vazante, hydrothermal alteration resulted in silicification and dolomite, siderite, jasper, hematite, and chlorite formation. These processes were accompanied by strong relative gains of SiO₂, Fe₂O_3(T), Rb, Sb, V, U, and La, which are typically associated with the nonsulfide zinc mineralization. All sulfide-rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences.Oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the Vazante deposit, which is not a notable feature in the sulfide-rich deposits. This pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water, which may control the precipitation of the Vazante nonsulfide ore. Sulfide deposition resulted from fluid–rock interaction processes and mixing between the ascending metalliferous fluids and sulfur-rich tectonic brines derived from reduced shale units.     

The response of the Gulf of Mexico to wind and heat flux forcing: What has been learned in recent years?

The Loop Current and its shed eddies dominate the circulation and dynamics of the Gulf of Mexico (GoM) basin. Those eddies are strongly energetic and are the cause of intense currents that may penetrate several hundred meters deep. However, there are regions in the GoM and periods of time in which the local atmospheric forcing plays an important role in its dynamics and thermodynamics. The circulation on the shelves, and particularly on the inner shelf, is mainly wind-driven with seasonality, changing direction during the year with periods of favorable upwelling/downwelling conditions. The wind-driven circulation is associated with the transport of waters with different temperature and salinity characteristics from one region to another. The interannual variability of the circulation on the shelves is linked to the atmospheric variability. Intraseasonal variability of the wind patterns considerably affects the likelihood and magnitude of upwelling and downwelling. The geometry of the GoM is such that large-scale winds may drive opposing upcoast/downcoast currents along different parts of the curving coast, resulting in convergence or divergence zones. The width of the shelves in the GoM is variable;while the West Florida Shelf, the Texas-Louisiana shelf and the Campeche Bank are more than 200 km wide, they are narrower near Veracruz and Tabasco. Another consequence of the GoM physiography and the wind forcing is the development of cross-shelf transports in the southern Bay of Campeche, the southern Texas shelf and southeast of the Mississippi river, which in turn vary during the year. During autumn-winter (from September to April), the GoM is affected by cold fronts coming from the northwest United States, which are associated with strong, dry, and cold winds that mix its waters and generate large sensible and latent heat fluxes from the ocean to the atmosphere. These frontal passages also cool the GoM surface waters due to mixing with lower temperature subsurface waters. During summer, tropical cyclones crossing the GoM can dramatically affect circulation and coastal upwelling.     

Calcium Isotopic Fractionation during Ion‐Exchange Column Chemistry and Thermal Ionisation Mass Spectrometry (TIMS) Determination

Significant isotopic fractionation can occur during column chemistry and determination by mass spectrometry. Improper correction may produce uncertainties in the isotopic composition of geological samples. We investigated calcium isotopic fractionation during these two processes and set up a model to check data quality. The δ^44/40Ca_915a value of IAPSO seawater in different Ca cuts (e.g., 0–20, 20–40, 40–60, 60–80 and 80–100%) on column chemistry ranged from ~ 4‰ to 0‰. The more Ca was eluted, the lower the δ^44/40Ca_915a value of the elution was found. The isotopic fractionation of calcium on the column appeared to follow the exponential law. However, TIMS instrumental fractionation during Ca runs did not always follow the exponential law due to mixing effects from sample reservoirs on the filament. Our results show that errors could be caused if the instrumental fractionation deviates from the exponential law, especially when the fractionation degree is large. To improve the measurement uncertainty, a model is proposed to check the behaviour and degree of instrumental fractionation, which will provide a quick and reasonable verdict on the data quality of TIMS runs.     

湘南宝山矿床花岗闪长斑岩中黑云母的矿物学特征及其指示意义

湖南宝山矿床处于坪宝矿带的北端,是湘南地区最大的铜多金属矿床。为了进一步探讨矿区内花岗质岩石的形成条件及成矿潜力,文章在详细的野外地质和岩相学观察的基础上,对与成矿密切相关的花岗闪长斑岩中的黑云母进行了详细的矿物化学分析。电子探针分析结果表明:宝山花岗闪长斑岩中的黑云母为铁质黑云母和镁质黑云母,其中,Ti介于0.18~0.30,且Mg/(Mg+Fe2+)值介于0.42~0.58,属于典型的岩浆成因黑云母;黑云母的氧化系数为0.16~0.26,w(Mg O)为8.17%~11.72%,平均9.3%,MF值范围为0.38~0.50,指示其岩体属于壳幔混源型的I型花岗岩;岩体中以黑云母的全铝含量计算的结晶压力为97~174 MPa,相应的结晶深度为3.67~6.57 km,平均深度为5.12 km。其log f(O2)变化范围为-14.5~-12.8,表明黑云母是在较高氧逸度条件下结晶形成的,有利于铜矿的形成。     

Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ¹³C, δ¹⁸O and δ²⁶Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH_NBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.