Sedimentology and geochemistry of ODP Leg 129 siliceous deposits

Siliceous deposits drilled on Ocean Drilling Program Leg 129 accumulated within a few degrees of the equator during the Jurassic through early Tertiary, as constrained by paleomagnetic data. During the Jurassic and Early Cretaceous, radiolarian ooze, mixed with a minor amount of pelagic clay, was deposited near the equator, and overall accumulation rates were moderate to low. At a smaller scale, in more detail, periods of relatively higher accumulation rates alternated with periods of very low accumulation rates. Higher rates are represented by radiolarite and limestone; lower rates are represented by radiolarian claystone. Our limited data from Leg 129 suggests that accumulation of biogenic deposits was not symmetrical about the equator or consistent over time. In the Jurassic, sedimentation was siliceous; in the Cretaceous there was significant calcareous deposition; in the Tertiary claystone indicates significantly lower accumulation rates at least the northern part of the equatorial zone. Accumulation rates for Leg 129 deposits in the Cretaceous were higher in the southern part of the equatorial zone than in the northern part, and the southern side of this high productivity zone extended to approximately 15°S, while the northern side extended only to about 5°N.
Accumulation rates are influenced by relative contributions from various sediment sources. Several elements and element ratios are useful for discriminating sedimentary sources for the equatorial depositional environments. Silica partitioning calculations indicate that silica is dominantly of biogenic origin, with a detrital component in the volcaniclastic turbidite units, and a small hydrothermal component in the basal sediments on spreading ridge basement of Jurassic age at Site 801. Iron in Leg 129 sediments is dominantly of detrital origin, highest in the volcaniclastic units, with a minor hydrothermal component in the basal sediments at Site 801. Manganese concentrations are highest in the units with the lowest accumulation rates. Fe/Mn ratios are >3 in all units, indicating negligible hydrothermal influence. Magnesium and aluminum concentrations are highest in the volcaniclastic
units and in the basal sediments at Site 801. Phosphorous is very low in abundance and may be detrital, derived from fish parts. Boron is virtually absent, as is typical of deep-water deposits. Rare earth element concentrations are slightly higher in the volcaniclastic deposits, suggesting a detrital source, and lower in the rest of the lithologic units. Rare earth element abundances are also low relative to "average shale." Rare earth element patterns indicate all samples are light rare earth element enriched. Siliceous deposits in the volcaniclastic units have patterns which lack a cerium anomaly, suggesting some input of rare earth elements from a detrital source; most other units have a distinct negative Ce anomaly similar to seawater, suggesting a seawater source, through adsorption either onto biogenic tests or incorporation into authigenic minerals for Ce in these units.
The Al/(Al + Fe + Mn) ratio indicates that there is some detrital component in all the units sampled. This ratio plotted against Fe/Ti shows that all samples plot near the detrital and basalt end-members, except for the basal samples from Site 801, which show a clear trend toward the hydrothermal end-member. The results of these plots and the association of high Fe with high Mg and Al indicate the detrital component is dominantly volcaniclastic, but the presence of potassium in some samples suggests some terrigenous material may also be present, most likely in the form of eolian clay. On Al-Fe-Mn ternary plots, samples from all three sites show a trend from biogenic ooze at the top of the section downhole to oceanic basalt. On Si-Fe-Mn ternary plots, the samples from all three sites fall on a trend between equatorial mid-ocean spreading ridges and north Pacific red clay. Copper-barium ratios show units that have low accumulation rates plot in the authigenic field, and radiolarite and limestone samples that have high accumulation rates fall in the biogenic field.

本研究依托大洋钻探计划第129航次（Ocean Drilling Program Leg 129）钻取的硅质沉积岩芯，经古地磁数据约束，其沉积区域位于赤道附近几度范围内，沉积时代为侏罗纪至早第三纪。侏罗纪至早白垩世时期，赤道附近沉积了放射虫软泥，混杂少量远洋黏土，整体沉积速率处于中低水平。在更小尺度的精细分析中，沉积速率较高的时期与极低沉积速率的时期交替出现：高沉积速率阶段以放射虫岩和石灰岩为沉积标识，低沉积速率阶段则以放射虫黏土岩为代表。本航次有限的钻探数据表明，生物成因沉积的堆积并不以赤道为对称中心，且随时间呈现显著变化。侏罗纪时期沉积以硅质成分为主；白垩纪时期则发育大量钙质沉积；第三纪时期，至少赤道带北部区域的黏土岩指示沉积速率显著降低。白垩纪时期，第129航次沉积的堆积速率在赤道带南部高于北部；该高生产力带的南界延伸至约南纬15°，而北界仅延伸至约北纬5°。

沉积速率受不同沉积物源的相对贡献量影响。多种元素及其比值可用于判别赤道沉积环境的沉积物来源。硅元素分配计算结果显示，硅质组分主要为生物成因；火山碎屑浊积岩单元中存在陆源碎屑组分，而801站位（Site 801）侏罗纪洋中脊基底之上的底部沉积物中则含有少量热液成因组分。第129航次沉积物中的铁主要为陆源碎屑成因，在火山碎屑岩单元中含量最高，仅在801站位的底部沉积物中存在少量热液成因铁。锰元素含量在沉积速率最低的岩性单元中最高；所有岩性单元的Fe/Mn比值均大于3，表明热液作用影响可忽略不计。镁、铝元素含量在火山碎屑岩单元及801站位的底部沉积物中最高。磷元素丰度极低，其来源可能为陆源碎屑，或源自鱼类遗骸。硼元素几乎完全缺失，这与深水沉积的典型特征一致。稀土元素浓度在火山碎屑沉积物中略高于其他岩性单元，指示其陆源碎屑成因；而整体稀土元素丰度相较于"average shale"仍偏低。稀土元素配分模式显示，所有样品均富集轻稀土元素。火山碎屑岩单元中的硅质沉积无铈异常，表明其稀土元素有部分来自陆源碎屑输入；其余多数岩性单元则呈现与海水相似的显著负铈异常，指示铈元素来自海水——具体通过吸附在生物壳体之上，或是进入自生矿物晶格中实现。

Al/(Al+Fe+Mn)比值表明，所有被采样的岩性单元中均存在陆源碎屑组分。以该比值为横轴、Fe/Ti比值为纵轴的图解显示，除801站位的底部样品呈现向热液端元演化的清晰趋势外，其余所有样品均集中在陆源碎屑与玄武岩端元附近。上述图解结果，以及高Fe与高Mg、Al的元素组合特征，均表明陆源碎屑组分主要为火山碎屑成因；但部分样品中钾元素的存在则指示可能同时存在陆源物质，其最可能的赋存形式为风成黏土。在Al-Fe-Mn三元图解中，三个站位的样品均呈现从剖面顶部的生物成因软泥向下逐渐演变为洋壳玄武岩的趋势。在Si-Fe-Mn三元图解中，所有三个站位的样品均落在赤道洋中脊与北太平洋红黏土之间的演化趋势线上。Cu/Ba比值图解显示，低沉积速率的岩性单元落在自生成因区域，而高沉积速率的放射虫岩与石灰岩样品则落入生物成因区域。