江汉平原47号钻孔中的化石硅藻及其在古环境分析上的意义

江汉平原47号钻孔,位于湖北省江汉平原的江陵县,在其上段的46.6-2.2m层位上,有丰富的化石硅藻,约有26属148种,其中绝大多数是现存的淡水种类,并以附生性、沿岸带和浅水性的种类为主。种类较多的属有:Navicula(28种)、Cymbella(23种)、Achnanthes(15种)、Gomphonema(14种)和Eunotia(13种)。主要种类有Gomphonema tropicale var. nonpunctatum、Cyclotella comta、Cocconeis placemula vat. lineata和Stephanodiscus dubius等。硅藻中的种数(种/层)和数量(个/每克干沉积物)经历了一个从无到有、由少到多再逐渐减少直至消失的过程。这反映了水体环境也相应地经历了一个漫长的变迁过程:河流(可能是长江古道)→边滩和河漫滩→泛滥平原的积水洼地(湖泊或沼泽)→水体变深加宽→水体消失。从不同沉积相中各种类型(适酸碱度不同)的硅藻比例(即硅藻种谱),可以推测当时水体的酸碱度的变化不大,在中性或中性偏碱的范围。由化石硅藻并结合孢粉的矿物分析,可以推测当时硅藻大量生长繁殖时期的水温较凉,气候可能偏冷。硅藻的数量与沉积物的平均粒度呈明显的相关性,硅藻的种数与沉积物的平均粒度也呈明显的相关性。

FOSSIL DIATOMS IN NO.47 BOREHOLE OF JIANGHAN PLAIN AND THEIR SIGNIFICANCE TO PALEOENVIRONMENTAL ANALYSIS

The No. 47 borehole is located at Jiangling County in the Jianghan Plain, Hubei Province, China. There are abundant sedimentary diatoms (about 26 genera and 148 taxa) in the upper section (46.6-2.2m) of the borehole. Most of them belong to the existent freshwater diatoms, being mainly epiphytic, littoral and shallow-water-inhabitant species. Among them some gerera are rich in taxa such as Navicula (28 taxa), Cymbella (23 taxa), Achnanthes ( 15 taxa), Gomphonema (14 taxa) and Eunotia (13 taxa) .The dominant species are Gomphonema tropicale var. nonpuncta-tum, Cyclotella comta, Cocconeis placentula var. lineata and Stephanodiscus dubius, etc. From the lower to the upper section of the sediment, species number (Sp. /strat.) and amount (Ind. /g dry sed.) of the diatoms underwent a change from scantiness to plenty, then gradually decline to final disappearance. The change of the diatoms obviously reflects a paleoenvironmental change of water bodies in this area,i.e. :at the early stage,this area was possibly a river (might have been an obsolete course of the Yangtze River), then it gradually became a side bank in the river, which later turned into a depression retaining water perphaps in the form of a lake or a swamp in a flood plain. Then it gradually became deepened and widened with final disappearance of the retained water. According to the diatom assemblages in different sedimentary phases, it may be inferred that the pH range in these water bodies were basically stable, ranging from neutral (pH = 7) to slightly alkaline (pH>>>>>>>7) .Combining pollen analysis (mainly the pollens of Pinus and Betule) and mineral analysis with diatoms, it might be deduced that the paleoclimate of this area during the period of luxuriant growth of diatoms was somewhat colder than the present time, In the sediment, the amount and species number of the diatoms all significantly correlated with the mean grain size of the sediment.