Size-specific opal-bound nitrogen isotope measurements in North Pacific sediments
Size-specific opal-bound nitrogen isotope measurements in North Pacific sediments
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北太平洋沉积物中特定尺寸的蛋白石结合氮同位素测量
DOI：
10.1016/j.gca.2013.06.041
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发表时间：
2013
影响因子：
5
通讯作者：
G. Haug
中科院分区：
文献类型：
--
作者：
A. Studer;K. Ellis;S. Oleynik;D. Sigman;G. Haug
硅藻蛋白石的氮同位素组成（δ 15 Ndb）是极地表层海洋硝酸盐利用的有价值记录，也是生物泵效率的量度。过去对δ 15 Nd的岩芯记录中，生物蛋白石的测量值最大可达150 μm，这应该代表了大部分保存下来的硅藻，但也可能包括非硅藻蛋白石，如放射虫和海绵骨针。本研究将亚北极太平洋和白令海全新世至末次冰期沉积物中的蛋白石分离成不同的粒级，测定其δ 15 N值。我们发现，δ 15 N在所有站点和通过时间随尺寸增加而减小的总体趋势。显微镜检查涂片和分析蛋白石的显微照片的图像面积分析显示，较大的尺寸级分包含更大比例的海绵骨针和放射虫。人工分离和测量海绵骨针的结果显示，它们的δ 15 N值非常低（δ-11 ‰）。由于这些相对较大的化石破碎，样品制备过程中的超声处理导致更大的骨针和放射虫污染，导致δ 15 Ndb随着尺寸的增加而急剧下降，并且超声处理与非超声处理样品中所有尺寸级分的δ 15 Ndb均较低。然而，这些污染物也存在，尽管在未经超声处理分离的样品的各种尺寸级分中含量较低，并且这些样品还显示出δ 15 Ndb随着尺寸的增加而降低。白令海全新世沉积物的简单同位素质量平衡计算表明，大粒级沉积物中δ ~（15）Ndb的变化可以用各粒级中低δ ~（15）N海绵骨针的相对丰度来解释。然而，一些尺寸分数δ 15 N的差异在井下记录需要不同的解释。硅藻的种间或种内效应都是明显的，表明较大的（中生）硅藻比较小的（羽生）硅藻的δ 15 Ndb低，且δ 15 Ndb随中生硅藻硅藻细胞壳尺寸的增加而降低。非硅藻蛋白石对氮的污染通常不会影响硅藻结合的δ 15 N（0-150 μm），因为非硅藻蛋白石通常占总蛋白石的比例小于0.5%。然而，在亚北极北太平洋的早期冰消期（Heinrich Stadial 1-相关）是一个重要的可能例外：其低浓度蛋白石的相当大一部分似乎是海绵骨针和放射虫，因此重建的总硅藻δ 15 Ndb下降在这个时候可能是一个人为因素。虽然冰川期沉积物也容易受到非硅藻污染，但这应该会降低δ 15 Ndb，因此北太平洋沉积物中观察到的高冰川δ 15 Ndb不能用污染来解释。因此，先前关于北太平洋区域在最后一个冰河期营养消耗增加的解释仍然有效。
The nitrogen isotope composition of diatom opal (δ15Ndb) is a valuable recorder of nitrate utilization in the polar surface oceans and a measure of the efficiency of the biological pump. Past down-core records of δ15Ndbinvolved the measurement of the biogenic opal fraction up to 150 μm in size, which should represent the bulk of the preserved diatoms but may also include non-diatom opal such as radiolaria and sponge spicules. In this study, the opal from subarctic Pacific and Bering Sea sediments from the Holocene back to the last glacial was separated into different size fractions to measure their individual δ15N. We found a general trend of decreasing δ15N with increasing size at all sites and through time. Microscopic investigation of smear slides and image area analysis of microphotographs of the analyzed opal revealed that the larger size fractions contained greater proportions of sponge spicules and radiolaria. Manual isolation and measurement of the sponge spicules showed that they have a very low δ15N (∼ −11‰). Ultrasonication during sample preparation caused greater spicule and radiolaria contamination due to fragmentation of these relatively large fossils, leading to a dramatic δ15Ndbdecrease with increasing size and lower δ15Ndbacross all size fractions in sonicated versus non-sonicated samples. Nevertheless, these contaminants were also present albeit less abundant in the various size fractions of samples separated without sonication, and these samples also showed a δ15Ndbdecrease with increasing size. Simple isotope mass-balance calculations of Holocene Bering Sea sediments indicate that most of the δ15Ndbvariations among the larger size fractions can be explained by the relative abundance of low-δ15N sponge spicules in each fraction. However, some of the size fraction δ15N differences in the downcore records require a different explanation. Both diatom inter- or intra-species effects are evident and indicate lower δ15Ndbamong the larger (centric) versus smaller (pennate) diatom species and a δ15Ndbdecrease with increasing size of centric diatom frustules.Contamination of N by non-diatomaceous opal should not normally compromise total-diatom-bound δ15N (0–150 μm) because the non-diatom opal typically contributes less than ∼5% to the total opal. However, the early deglacial (Heinrich Stadial 1-correlative) period in the subarctic North Pacific is an important possible exception: a substantial fraction of its low concentration of opal appears to be sponge spicules and radiolaria, such that the reconstructed total-diatom δ15Ndbdecrease at this time may be an artifact. While the glacial-age sediments are also vulnerable to non-diatom contamination, this should work to lower δ15Ndb, such that the observed high glacial δ15Ndbin North Pacific sediments cannot be explained by contamination. Thus, the previous interpretation of enhanced nutrient consumption in the North Pacific regions during the last ice age remains valid.
影响因子：
56.9
作者：
Ren, H.;Sigman, D. M.;Haug, G. H.
通讯作者：
Haug, G. H.