Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo-proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the d15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (d15N[NOx]) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping d15NNOx values relatively low in the subsurface waters. However d15N[NOx] values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. d15N[sed] is consistently lower than the isotopic signature of upwelled [NO3]-, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled [NO3]- are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface d15N[NOx] values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north-south gradient towards stronger surface [NO3]- utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, d15N[sed] is lower than maximum water column d15N[NOx] values most likely because only a portion of the upwelled water originates from the depths where highest d15N[NOx] values prevail. Though the enrichment of d15N[NOx] in the subsurface waters is unambiguously reflected in d15N[sed] values, the magnitude of d15N[sed] enrichment depends on both the depth of upwelled waters and high subsurface d15N[NOx] values produce by N-loss. Overall, the degree of N-loss influencing subsurface d15N[NOx] values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo d15N[sed] records from the Peruvian oxygen minimum zone.

本研究报道了采集自北纬1°至南纬18°间秘鲁与厄瓜多尔陆缘的水柱及表层沉积物的新型氮同位素数据，用于古代用指标验证。在现今秘鲁氧最低区（oxygen minimum zone, OMZ）内外的80余个采样点位，我们测定了全沉积物中的生产力代用指标（有机碳、总氮、生物蛋白石、C37烯酮浓度）以及15N/14N比值。缺氧条件下，次表层水体中微生物介导的氮损失过程（将氮转化为氮气）会在下方沉积物中留下特征性的15N富集信号。研究同时发现，秘鲁上升流系统高营养低叶绿素（high nutrient, low chlorophyll, HNLC）区域的表层水体中，浮游植物的营养盐摄取过程同样会对沉积信号产生影响。为明确沉积δ15Nsed信号与上述过程的关联，我们将岩芯表层的同位素值与水柱上层200 m内硝酸盐、亚硝酸盐的δ15N（记为δ15N[NOx]）进行了对比分析。在北纬1°至南纬10°区间，次表层水体的氧浓度仍足够高，可抑制氮损失过程，使得次表层水体的δ15NNOx值维持在较低水平。但在该HNLC区域，透光带内部分硝酸盐被浮游植物摄取，导致表层水体的δ15N[NOx]值随深度变浅而升高。沉积δ15Nsed值始终低于上升流携带的[NO3]-的同位素特征，这大概率源于同期形成的有机质具有15N贫化效应。在南纬10°至15°区间，该区域为常年上升流单元的当前分布位置，HNLC条件有所缓解，生物生产及近表层浮游植物对上升流携带的[NO3]-的摄取作用均达到最强。此外，次表层氧浓度降低至足以支持反硝化作用与厌氧氨氧化（anammox）介导的氮损失过程，使得沿岸上升流的源水中次表层δ15N[NOx]值升高。沉积物中的各类生产力代用指标均反映出向南纬方向生产强度逐渐升高的趋势；而南北向梯度——即表层[NO3]-摄取作用增强与次表层氮损失过程加剧——则体现在表层沉积物的15N/14N比值变化中。在南纬10°以南区域，沉积δ15Nsed值低于水柱中最高的δ15N[NOx]值，这极有可能是因为仅部分上升流水体来自δ15N[NOx]值最高的深层水体。尽管次表层水体中δ15N[NOx]的富集过程可在沉积δ15Nsed值中得到明确反映，但沉积δ15Nsed的富集幅度同时取决于上升流水体的起源深度，以及氮损失过程产生的高次表层δ15N[NOx]值。总体而言，在解释秘鲁氧最低区的古沉积δ15Nsed记录时，需综合考虑影响次表层δ15N[NOx]值的氮损失程度、上升流水体的起源深度，以及HNLC条件下近表层硝酸盐的摄取程度。