Properties of foraminifera from Denmark Strait

The Denmark Strait is a key area for the exchange of water masses and functions as a deep-water gateway between the Nordic Seas and the North Atlantic. Thus it constitutes an important part of the global thermohaline circulation (THC). The gateway aperture is influenced by eustatically and isostatically controlled changes of the Denmark Strait sill and by alteration in density contrasts between the water masses in North and South. In order to determine the faunal exchange through the changing aperture, benthic foraminiferal assemblages were analysed. Eight surface sediment samples were taken along the East Greenland continental margin during the RV Polarstern cruise ARK-XVIII/1 in July 2002 by means of a multicorer and a box corer respectively. Faunal assemblages of living (stained) and dead benthic foraminifera north and south of the Denmark Strait (980-2564 m water depth) were compared with each other. Northern and southern stations exhibit clear differences in abundances, diversities, number of species, and species composition. Due to strong bottom currents and low carbonate dissolution rates, high abundances with predominating calcareous species of dead and evidently allochthonous foraminifera were recorded south of the sill. Rather low abundances of dead autochthonous foraminiferal assemblages, containing mainly agglutinated species, were found north of the sill because of weak bottom currents and high carbonate dissolution rates. Southern stations showed a large portion of epibenthic species living as suspension feeders. In contrast endobenthic species living as substrate feeders dominated northern stations, pointing to lateral advection of food particles in South and sedimentation of food particles in North. Morphomertic measurements were performed on the high abundant foraminifer Cibicidoides wuellerstorfi. In order to supplement the data of the surface sediment samples and obtain morphometric data of glacial specimens, two long sediment cores were investigated as well. These cores were taken during the RV Polarstern cruise ARK-XVIII/1 north of the Denmark Strait and the RV Poseidon cruise PO210/2 in August 1995 south of the sill. Northern glacial samples contained no C. wuellerstorfi. The morphometric investigations included counting of chambers within the last whorl, determination of the maximum test diameter, identification of the coiling direction, measurement of the proloculus (initial chamber) diameter, and weighing of individual tests. It was statistically significant that chamber numbers decreased form glacial over northern to southern stations. Maximum diameters, percentages of coiling direction, proloculus diameters, and weight of single tests showed no differences. Evidences of a faunal exchange over the sill lead to the conclusion that C. wuellerstorfi from both sides of the sill response to diverse environmental conditions and develop two different ecophenotyes rather than genotypes. For C. wuellerstorfi the Denmark Strait is more likely a migration passage than a barrier.

丹麦海峡是水团交换的关键区域，同时作为北欧海与北大西洋之间的深水通道，因此是全球温盐环流（thermohaline circulation, THC）的重要组成部分。该通道的开口大小受海面升降与地壳均衡作用调控的丹麦海峡海槛变化，以及南北两侧水团密度差的改变共同影响。为探究开口变化过程中的生物群交换，研究人员对底栖有孔虫组合（benthic foraminiferal assemblages）开展了分析。2002年7月，搭载极星号（RV Polarstern）的ARK-XVIII/1科考航次中，研究人员分别利用多管采样器（multicorer）与箱式采样器（box corer），沿东格陵兰大陆边缘采集了8个表层沉积物样品。对丹麦海峡南北两侧（水深980~2564米）的活体（染色）与死亡底栖有孔虫的生物群组合进行了对比分析。南北站位在丰度、多样性、物种数量及物种组成上均存在显著差异。由于海峡南侧存在强底流且碳酸盐溶解速率较低，研究记录到较高丰度的死亡有孔虫，且以钙质物种为主，其中部分明显为异地搬运类群。海峡北侧则因底流较弱、碳酸盐溶解速率较高，仅发现丰度较低的死亡原地有孔虫组合，其类群以胶结质壳物种为主。南侧站位以悬浮食性的表生底栖物种占比更高；与之相反，北侧站位以底质食性的内生底栖物种为主，这表明南侧存在食物颗粒的侧向平流，而北侧则发生食物颗粒的沉积作用。针对丰度较高的有孔虫类群伍氏似卷虫（Cibicidoides wuellerstorfi），研究人员开展了形态测量分析。为补充表层沉积物样品的数据并获取冰川期标本的形态测量数据，研究同时对两根长沉积岩芯进行了分析。这两根岩芯分别由极星号ARK-XVIII/1航次在丹麦海峡北侧采集，以及波塞冬号（RV Poseidon）PO210/2航次于1995年8月在海峡南侧采集。北侧的冰川期样品中未发现伍氏似卷虫。形态测量研究内容包括：统计最后一壳圈的壳室数量、测定壳体最大直径、识别旋向、测量初房（proloculus）直径，以及单个壳体的称重。统计分析显示，壳室数量从冰川期样品、北侧站位到南侧站位呈显著下降趋势。而壳体最大直径、旋向占比、初房直径及单个壳体重量均未表现出显著差异。基于海峡两侧存在生物群交换的证据，研究得出结论：海峡两侧的伍氏似卷虫响应于不同的环境条件，演化出两种不同的生态表型而非遗传型。对于伍氏似卷虫而言，丹麦海峡更像是一个迁移通道而非阻隔屏障。