NOAA/WDS Paleoclimatology - Torfdalsvatn, Iceland Geochemistry and Magnetic Susceptibility Data during the Holocene

Open questions remain around the Holocene variability of climate in Iceland, including the relative impacts of natural and anthropogenic factors on Late Holocene vegetation change and soil erosion. The lacustrine sediment record from Torfdalsvatn, north Iceland, is the longest known in Iceland (~12000 cal yr BP), and along with its high sedimentation rate, it provides an opportunity to develop high-resolution quantitative records that address these challenges. In this study, we use two sediment cores from Torfdalsvatn to construct a detailed age model derived from marker tephra layers, paleomagnetic secular variation, and radiocarbon (20 age control points). We then apply this robust age constraint to support a complete tephrochronology (> 2200 grains analyzed in 33 tephra horizons) and sub-centennial geochemical (MS, TOC, C/N, δ13C, and BSi) and algal pigment records (chlorins, lutein, diatoxanthin, and canthaxanthin). Along with previously published proxy records from the same lake, these records demonstrate generally stable terrestrial and aquatic conditions during the Early and Middle Holocene, except for punctuated disturbances linked to major tephra fall events. During the Late Holocene, there is strong evidence for naturally driven algal productivity decline beginning around 1800 cal yr BP. These changes closely follow regional Late Holocene cooling driven by decreases in Northern Hemisphere summer insolation and the expansion of sea-ice-laden Polar Water around Iceland. Then at 880 cal yr BP, ~200 years after the presumed time of human settlement, a second shift in the record begins and is characterized by a strong uptick in landscape instability and possibly soil erosion. Collectively, the Torfdalsvatn record highlights the resilience of low-elevation, low-relief catchments to the pre-settlement soil erosion in Iceland, despite a steadily cooling background climate. The precisely dated, high-resolution tephra and paleoenvironmental record from this site can serve as a regional template for lowland regions of north Iceland.

目前冰岛全新世气候变率仍存在诸多待解问题，其中包括自然与人为因素对全新世晚期植被变化及土壤侵蚀的相对影响。冰岛北部托夫达尔斯湖（Torfdalsvatn）的湖相沉积记录是冰岛已知最长的沉积记录（约12000校准年BP，cal yr BP），加之其沉积速率较高，为构建高分辨率定量记录以解决上述难题提供了契机。本研究依托采自托夫达尔斯湖的两根沉积物岩芯，基于标志火山灰层、古地磁长期变化（paleomagnetic secular variation）以及放射性碳测年（20个年代控制点）构建了高精度年代模型。随后，我们借助这一可靠的年代约束条件，完成了完整的火山灰年代学（tephrochronology）研究（在33个火山灰层位中分析了超过2200颗颗粒），并获取了亚百年尺度的地球化学记录——磁化率（Magnetic Susceptibility，MS）、总有机碳（Total Organic Carbon，TOC）、碳氮比（C/N）、碳同位素δ¹³C以及生物硅（Biogenic Silica，BSi）——与藻类色素记录（叶绿素类物质、叶黄素、硅藻黄质以及角黄素）。结合该湖泊此前已发表的代用指标记录，上述记录显示，全新世早中期陆地与水生环境整体处于稳定状态，仅在重大火山灰沉降事件引发的间断性扰动时期出现异常。全新世晚期则有充分证据表明，约1800校准年BP起，藻类生产力出现自然驱动的下降趋势。这一变化紧随区域全新世晚期降温过程发生，该降温由北半球夏季日照量减少以及冰岛周边覆海冰极地水团扩张所驱动。而在880校准年BP时，也就是人类定居冰岛的推定时间之后约200年，记录出现第二次转变，其特征为地表环境不稳定性显著上升，且可能伴随土壤侵蚀加剧。综合来看，托夫达尔斯湖的沉积记录表明，尽管背景气候持续变冷，但冰岛低海拔、低起伏汇水区在人类定居前的土壤侵蚀过程中仍表现出较强的恢复力。本位点精准定年的高分辨率火山灰与古环境记录，可作为冰岛北部低地地区的区域古环境研究模板。