颗石藻与海洋、环境关系之探讨

笔者在20年的研究中相继发现,自中元古代和古生代以来海相钙质超微浮游生物－颗石藻的新属种达180余种,通过对颗石藻在海洋中分布状态与固碳特征的研究,证明颗石藻乃是海洋中重要的基础生产者,它和海洋碳酸盐及相关生物存在着密切的关系,其能借助光合作用,吸收海水中的CO2,而释放出氧气．对减少全球温室效应与保护地球生态环境平衡,有相当重要的贡献。     

Magnetic susceptibility variations of the Ediacaran cap carbonates in the Yangtze platform and their implications for paleoclimate

Magnetic susceptibility (MS) data were obtained from 11 sections of the Doushantuo (Ediacaran) cap carbonate that directly overlies the Nantuo glacial diamictite in the southeastern margin of the Yangtze platform. The MS data revealed two regionally correlatable peaks at the bottom and top of the cap carbonate, separated by an interval of low values. The lower MS peak coincides with high percentage of insoluble siliciclastic residues that are compositionally identical to the matrix of the underlying diamictite, suggesting its origin controlled mainly by detrital components during the first phase of cap carbonate deposition at the end of the glaciation. The upper MS peak is associated with high clay content and iron sulfides, and can be interpreted as either derived from enhanced greenhouse weathering that could have brought more terrigenous components into the ocean, or the result of ocean anoxia at the late stage of cap carbonate deposition that could led to formation of abundant iron sulfides. The regionally consistent MS curves from the cap carbonates provided the first geophysical record for the rapid climate change from icehouse to greenhouse conditions in the aftermath of the Neoproterozoic “snowball Earth” event.     

Rhodolith-bearing limestones as transgressive marker beds: fossil and modern examples from North Island, New Zealand

Rhodoliths are nodular structures composed mainly of the superimposed thalli of calcareous red algae. Because their development is controlled by an array of ecological parameters, rhodoliths are a valuable source of palaeoenvironmental information. However, despite their common use in palaeoecological reconstructions, the stratigraphic significance of rhodolith accumulations seldom has been addressed in detail. In a study of Cenozoic rhodolith‐bearing deposits from the North Island of New Zealand, rhodolithic units, usually of limited lateral extent, typically occur above major unconformities at the base of deepening upwards successions. Two types of transgressive rhodolith‐bearing deposits may be distinguished on the basis of texture and rhodolith internal structure: (i) type A deposits are clast‐supported rhodolithic rudstones containing abundant pebbles and cobbles reworked from the substrate, and are characterized by rhodoliths with a compact concentric to columnar internal structure and a high nucleus to algal cover ratio; (ii) type B deposits are rhodolithic floatstones with a matrix usually consisting of bryozoan fragments, benthic foraminifera and echinoid fragments or terrigenous silty fine sand. The rhodoliths of type B units usually have a loose internal framework with irregular to branched crusts. The two contrasting rhodolith‐bearing units are interpreted as characteristic facies of transgressive systems tract deposits, analogous to shell concentrations formed under conditions of low net sedimentation. Type A deposits are correlated with relatively high‐energy settings and/or narrow submerged palaeotopographic lows, whereas type B deposits are interpreted as forming in lower‐energy settings. The association between transgression and development of rhodolithic facies is confirmed by observations of a modern rhodolith production site at Whangaparaoa Peninsula in North Island, where algal nodules grow above a ravinement surface cut during the Holocene sea‐level rise, and also by a review of published fossil examples, many of which show stratigraphic and compositional attributes analogous to those of the New Zealand occurrences. The review indicates that transgressive rhodolith accumulations develop more commonly in, but are not restricted to, non‐tropical settings. It is suggested that a combination of factors, such as low net sedimentary input, nature of the substrate, sea‐level rise and inherited physiography contribute to determine the relationship between rhodolith‐bearing deposits and transgressive settings.     

Changes in Surface Morphology of Calcite Exposed to the Oceanic Water Column

Reactions occurring on the surfaces of biogenic carbonate minerals can have important consequences for the biogeochemical cycle of carbon. In this study, carbonate mineral surface reactions with ambient seawater were investigated by atomic force microscopy (AFM). A sampling method was developed in which calcite surfaces were hung at discrete depths on a sediment trap array line for a three-day deployment period in subtropical North Pacific waters. Changes in surface morphologies were examined at nanometer resolution and evaluated using as a constraint the depth profile of calcite saturation in these waters. Evidence suggests that: (1) organic films which develop on carbonate surfaces exposed to shallow seawater may be responsible for the oversaturated state of the upper oceanic water column, (2) dissolution of carbonate minerals within the shallow warm layer of the ocean could be responsible for part of the alkalinity anomaly observed in the North Pacific.     

Saddle dolomite and calcite cements as records of fluid flow during basin evolution: Paleogene carbonates,United Arab Emirates

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately ⁸⁷Sr-rich basinal NaCl–MgCl₂–H₂O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ¹⁸O_V-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ¹⁸O_V-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE–SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately ⁸⁷Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ¹⁸O_V-PDB and δ¹³C_V-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.     

Magnetostratigraphy of the Plio‐Pleistocene carbonate section of the Great Australian Bight

ODP Leg 182 drilled two north‐south transects in the western Great Australian Bight close to longitude 128°E. These transects penetrated a remarkably thick section of uppermost Pliocene and Pleistocene carbonate sediments, which are separated from Miocene sections by a major hiatus. In the eastern transect, at sites 1129, 1131 and 1127, the Brunhes‐Matuyama boundary (onset of C1n) was found at 343 m, between 280 and 300 m, and at 343 m below the sea floor, respectively. In the western transect, at sites 1132 and 1130, it was found between 170 and 181 m, and at 200 m, respectively. Within the Brunhes chron, inclination and intensity fluctuations and correlations between susceptibility and standard δ¹⁸O records were used to give age‐depth relations. These age relations from the palaeomagnetics are broadly consistent with the biostratigraphy and the δ¹⁸O results from Leg 182, but do not provide an entirely independent dataset. Rock magnetism stratigraphy at site 1131 revealed the principal bryozoan buildup to be at the time of the last glacial lowstand, as suggested by previous workers. At all sites the sedimentation rate increases from the basal unconformity up into the thick Upper Pleistocene section. In the western transect there appears to be a partial record of most of the Late Pliocene and Early Pleistocene chron boundaries (C1r1n to C2An3n) in a condensed section, but in the eastern transect only the Jaramillo (C1r1n) is observed.     

北京西山下苇甸第三统/芙蓉统界线附近碳酸盐岩地球化学特征及古环境意义

芙蓉世早期全球发生的生物绝灭事件与该时期古环境的变化可能存在着密切联系,但未有定论;相对于深水相区,浅水台地沉积物地球化学特征对古环境的变化有着更敏感的响应,有利于深化这一问题的理解。选取北京西山下苇甸剖面第三统崮山组和芙蓉统长山组碳酸盐岩沉积为研究对象,通过元素地球化学和同位素的分析,重建这一时期古环境的变迁过程,并探讨其与生命演化的关系。结果表明:崮山组和长山组沉积更替时期,古气候和古海洋环境有大的波动。海水由海平面低点快速海侵,水体还原性急剧增强,海水温度降至最低点,盐度升高至最高点,可能这是三叶虫生态崩溃及其灭绝的重要原因。长山组早期干热气候和温湿气候交替频繁,大规模海侵促使陆源碎屑物质输入的增加,海水中营养元素的富集促进了生物勃发;同时深水的还原性条件有利于有机质的保存,最终导致碳酸盐岩碳同位素的正漂。研究还表明,这次碳同位素正漂的发生与长山中晚期海平面的下降没有明显的成因联系。     

Brigantian and Middle Triassic remagnetizations in Lower Carboniferous carbonates, Northern Ireland: Role of diagenesis and fluid flow

Paleomagnetism (18 sites, 231 specimens) of Lower Carboniferous carbonates in Northern Ireland reveals three characteristic remanent magnetization (ChRM) components. Six sites from Brigantian limestones have a Middle Triassic (239 ± 7 Ma) secondary chemical remanent magnetization (CRM) in hematite, likely from alteration of the limestones by oxidizing meteoric fluids when continental red beds were deposited immediately above. Twelve sites from early Asbian limestones retain ChRM directions residing in pyrrhotite and magnetite. Their paleopoles are statistically indistinct, but suggest that the pyrrhotite remanence (326 ± 4 Ma) is about a million years younger than the magnetite remanence (327 ± 3 Ma). More importantly, the primary ChRM in these limestones was reset 3 or 4 Ma after deposition, probably by fluids involved in their diagenesis, giving secondary CRMs that are 8 Ma younger than those observed in the Lower Carboniferous carbonates that host the Navan Zn–Pb deposit in the Irish Midlands, suggesting two unrelated fluid histories.     

GasBenchⅡ顶空瓶内空气背景对<100μg碳酸盐中碳氧同位素在线测定的影响及校正方法初探

较为系统地研究了在特定排空时间下GasBenchⅡ顶空瓶内背景CO2的量和同位素组成变化,并检查了该空白CO2组分对碳酸盐C、O同位素在线测定的影响。结果发现,由于排空时间较短所导致的顶空瓶内剩余的空气CO2或由于排空方法不当所导致的外部空气少量回流都会对小样品量(100μg)碳酸盐C、O同位素测定结果产生显著影响。其影响程度取决于空白CO2的量和空白CO2与碳酸盐样品之间C、O同位素的差别大小。由此,对小样品量碳酸盐C、O同位素的测定结果进行了空白校正。当碳酸盐样品质量在20~100μg,校正后δ13C和δ18O值的标准偏差分别小于0.12‰和0.13‰,这证明了该空白校正方法的可行性。由于顶空瓶中空白CO2的量很小,所以对痕量CO2的量、δ13C和δ18O测定值的测定会存在一定波动,这对碳酸盐δ13C和δ18O校正值产生小于0.2‰的不确定度。采用准确测定的实验室空气CO2的δ13C和δ18O值则会大大减少该不确定度的影响。     

海相化学碳酸盐沉积中的铅同位素测定

铅同位素在沉积学和古海洋地质研究中具有广阔的应用前景 ,然而由于碳酸盐的含铅量甚微 ,给铅的化学分离、富集和同位素质谱分析带来困难 ,制约了这一领域的发展。本实验经过反复、深入探索 ,合理地制定了碳酸盐铅同位素样品制备的实验流程。通过对 2 2件灰岩、白云岩样品的铅同位素组成的系统测定 ,表明该实验流程稳定可靠。