Standing stocks and carbon isotopes of live benthic foraminifera from the South Atlantic

Live (Rose Bengal stained) and dead benthic foraminifera of surface and subsurface sediments from 25 stations in the eastern South Atlantic Ocean and the Atlantic sector of the Southern Ocean were analyzed to decipher a potential influence of seasonally and spatially varying high primary productivity on the stable carbon isotopic composition of foraminiferal tests. Therefore, stations were chosen so that productivity strongly varied, whereas conservative water mass properties changed only little. To define the stable carbon isotopic composition of dissolved inorganic carbon (d13CDIC) in ambient water masses, we compiled new and previously published d13CDIC data in a section running from Antarctica through Agulhas, Cape and Angola Basins, via the Guinea Abyssal Plain to the Equator. We found that intraspecific d13C variability of all species at a single site is constantly low throughout their distribution within the sediments, i.e. species specific and site dependent mean values calculated from all subbottom depths on average only varied by +/-0.09 per mil. This is important because it makes the stable carbon isotopic signal of species independent of the particular microhabitat of each single specimen measured and thus more constant and reliable than has been previously assumed. So-called vital and/or microhabitat effects were further quantified: (1) d13C values of endobenthic Globobulimina affinis, Fursenkoina mexicana, and Bulimina mexicana consistently are by between -1.5 and -1.0 per mil VPDB more depleted than d13C values of preferentially epibenthic Fontbotia wuellerstorfi, Cibicidoides pachyderma, and Lobatula lobatula. (2) In contrast to the Antarctic Polar Front region, at all stations except one on the African continental slope Fontbotia wuellerstorfi records bottom water d13CDIC values without significant offset, whereas L. lobatula and C. pachyderma values deviate from bottom water values by about -0.4 per mil and -0.6 per mil, respectively. This adds to the growing amount of data on contrasting cibicid d13C values which on the one hand support the original 1:1-calibration of F. wuellerstorfi and bottom water d13CDIC, and on the other hand document severe depletions of taxonomically close relatives such as L. lobatula and C. pachyderma. At one station close to Bouvet Island at the western rim of Agulhas Basin, we interpret the offset of -1.5 per mil between bottom water d13CDIC and d13C values of infaunal living Bulimina aculeata in contrast to about -0.6 +/- 0.1 per mil measured at eight stations close-by, as a direct reflection of locally increased organic matter fluxes and sedimentation rates. Alternatively, we speculate that methane locally released from gas vents and related to hydrothermal venting at the mid-ocean ridge might have caused this strong depletion of 13C in the benthic foraminiferal carbon isotopic composition. Along the African continental margin, offsets between deep infaunal Globobulimina affinis and epibenthic Fontbotia wuellerstorfi as well as between shallow infaunal Uvigerina peregrina and F. wuellerstorfi, d13C values tend to increase with generally increasing organic matter decomposition rates. Although clearly more data are needed, these offsets between species might be used for quantification of biogeochemical paleogradients within the sediment and thus paleocarbon flux estimates. Furthermore, our data suggest that in high-productivity areas where sedimentary carbonate contents are lower than 15 weight %, epibenthic and endobenthic foraminiferal d13C values are strongly influenced by 13C enrichment probably due to carbonate-ion undersaturation, whereas above this sedimentary carbonate threshold endobenthic d13C values reflect depleted pore water d13CDIC values.

本研究针对南大西洋东部及南大洋大西洋扇区25个站位的表层与次表层沉积物中，经罗丝孟加拉（Rose Bengal）染色的活体及死亡底栖有孔虫（benthic foraminifera）展开分析，旨在解析季节性与空间变化的高初级生产力对有孔虫壳体稳定碳同位素组成的潜在影响。为此，研究选取的站位生产力差异显著，但保守性水团性质变化极小。为确定周围水团中溶解无机碳（dissolved inorganic carbon, DIC）的稳定碳同位素比值（d13C_DIC），我们整合了从南极洲经厄加勒斯海盆、开普海盆、安哥拉海盆，经由几内亚深海平原至赤道的断面中的新获取及已发表的d13C_DIC数据。研究发现，单个站位内所有物种的种内d13C变异在其沉积物分布范围内始终处于较低水平：即从所有次表层深度计算得到的物种特异性及站位依赖性平均值，其平均变化幅度仅为±0.09‰。这一结果意义重大，因为它表明物种的稳定碳同位素信号不受所测单个标本的特定微生境影响，因此比此前假设的更为稳定可靠。本研究进一步量化了所谓的生命效应和/或微生境效应：(1) 内栖型Globobulimina affinis、Fursenkoina mexicana及Bulimina mexicana的d13C值，相较于优先表栖型Fontbotia wuellerstorfi、Cibicidoides pachyderma与Lobatula lobatula，始终较VPDB（维也纳佩迪贝石灰岩标准）偏轻1.5‰至1.0‰；(2) 与南极极锋区域不同，除非洲大陆坡上1个站位外，其余所有站位的Fontbotia wuellerstorfi的d13C值均无显著偏移地记录了底层水d13C_DIC值，而L. lobatula与C. pachyderma的d13C值分别较底层水值偏轻约0.4‰与0.6‰。这一发现进一步补充了关于有孔虫类群d13C值差异的日益增多的数据集：一方面支持Fontbotia wuellerstorfi与底层水d13C_DIC的原始1:1校准关系，另一方面也证实了其分类学近缘类群如L. lobatula和C. pachyderma存在显著的碳同位素偏轻现象。在厄加勒斯海盆西缘靠近布维岛的1个站位中，我们将底层水d13C_DIC与活体内栖型Bulimina aculeata的d13C值之间的-1.5‰偏移（邻近8个站位测得的偏移约为-0.6±0.1‰），解释为局域有机质通量与沉积速率升高的直接反映。此外，我们推测源自与大洋中脊热液活动相关的气体喷口释放的局域甲烷，可能是导致底栖有孔虫碳同位素组成发生显著13C贫化的原因。沿非洲大陆边缘，深内栖型Globobulimina affinis与表栖型Fontbotia wuellerstorfi之间，以及浅内栖型Uvigerina peregrina与F. wuellerstorfi之间的d13C值偏移，往往随有机质分解速率的整体升高而增大。尽管仍需更多研究数据，但这些物种间的偏移可用于量化沉积物内的生物地球化学古梯度，进而实现古碳通量估算。此外，本研究数据表明：在沉积物碳酸盐含量低于15%（质量分数）的高生产力区域，底栖有孔虫的d13C值易受13C富集作用的强烈影响，这可能源于碳酸根离子不饱和状态；而当沉积物碳酸盐含量高于该阈值时，内栖型有孔虫的d13C值则反映了孔隙水d13C_DIC的偏轻特征。