The aim of this project was to establish past variations in the main oceanographic and climatic features of a transitional semiarid ecosystem on the north-central Chilean coast. We analyzed recent sedimentary records retrieved from two bays, Guanaqueros and Tongoy (30^∘ S), for geochemical and biological analyses, including the following: sensitive redox trace elements, biogenic opal, total organic carbon (TOC), diatoms and stable isotopes of organic carbon and nitrogen. Three remarkable periods were established with different environmental conditions and productivities: (1) > 6600 cal BP, (2) 4500–1800 cal BP and (3) 140 cal BP to the present (2015 CE). The first period was characterized by a remarkably higher productivity (higher diatom abundances and opal) in which large fluxes of organic compounds were also inferred from the accumulation of elements, such as Ba, Ca, Ni, Cd and P, in the sediments. Meanwhile, significantly reduced conditions at the bottom of the bays were suggested based on the large accumulation of Mo, Re and U, showing a peak at 6600 cal BP, when sulfidic conditions could have been present. According to the pollen moisture index (PMI), this was also identified as the driest interval. These conditions should be associated with an intensification of the Southern Pacific Subtropical Anticyclone (SPSA) and stronger southerly western winds, emulating La Niña-like conditions, as has been described for the SE Pacific during the early Holocene and part of the mid-Holocene. During most of the second period, lower productivity was observed; however, a small increase was identified between 3400 and 4000 cal BP, although lower amounts of diatom (valves g⁻¹) and nutrient-type metal accumulations were evident. Anoxic conditions at the bottom of the bays changed to an almost stable suboxic condition during this time interval. The third period was marked by intense oxygenation after 1800 cal BP, as observed by a drastic change in the accumulation of U, Mo and Re. This was followed by a return to more reduced conditions over the past 2 centuries, characterized by a small productivity rise after ∼140 cal BP, as suggested by the opal accumulations. Overall, lower primary productivity, lower reduced conditions at the bottom and higher-humidity conditions were established after 6600 cal BP to the present. We suggest that the oxygenation might be associated with a weak effect from the oxygen minimum zone over the shelf and intensified El Niño activity, introducing oxygenated waters to the coastal zones through the propagation of equatorial waves and establishment of conditions that reduced the primary productivity from the mid-Holocene toward the beginning of the modern era.

中文翻译：

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重建过去约8000年智利中北部（30°S）以来环境条件和古生产力的变化

该项目的目的是确定智利中北部海岸过渡半干旱生态系统的主要海洋学和气候特征的过去变化。我们分析了从两个托架，Guanaqueros和Tongoy（30检索最近沉积记录^∘  S），地球化学和生物分析，包括以下情况：敏感氧化还原微量元素，生源蛋白石，总有机碳（TOC），硅藻和有机稳定同位素碳和氮。建立了具有不同环境条件和生产率的三个显着时期：（1）> 到现在（2015年CE）为6600千卡，（2）4500-1800千卡和（3）140千卡 第一阶段的特点是生产力显着提高（硅藻丰度和蛋白石更高），其中沉积物中Ba，Ca，Ni，Cd和P等元素的积累也推断出大量有机化合物通量。同时，由于Mo，Re和U的大量堆积，表明海湾底部的条件显着降低，这可能是在存在硫化物的条件下在6600 cal BP出现一个峰值。根据花粉水分指数（PMI），这也被确定为最干燥的间隔。这些条件应与南太平洋副热带反气旋（SPSA）的增强和更强的西南风相似，模拟类似拉尼娜的条件，正如在全新世早期和全新世中期的一部分中对东南太平洋的描述。在第二阶段的大部分时间里，观察到较低的生产率。但是，在3400至4000 cal BP之间发现有少量增加，尽管硅藻的量较低（阀门g^-1）和养分型金属积累明显。缺氧的在此时间间隔内，海湾底部的条件变为几乎稳定的亚氧条件。第三阶段的特征是在1800 cal BP后发生强烈的氧合作用，如U，Mo和Re积累的急剧变化所观察到的。随后在过去的两个世纪中又恢复了更为宽松的条件，其特征是在约140年后生产率小幅提高 蛋白石积聚所建议的cal BP。总体而言，在6600 cal BP到目前为止，建立了较低的初级生产力，较低的底部减压条件和较高的湿度条件。我们认为，充氧作用可能与来自陆架上最低限氧区的弱影响和厄尔尼诺现象加剧有关，它通过赤道波的传播和条件的建立将含氧水引入沿海地区，从而降低了主要生产力。到现代时代开始的全新世中期。