NOAA/WDS Paleoclimatology - Abrupt Holocene climate transitions in the northern North Atlantic region recorded by synchronized lacustrine records in Iceland

Two high-sediment-accumulation-rate Icelandic lakes, the glacial lake Hvítárvatn and the non-glacial lake Haukadalsvatn, contain numerous tephra layers of known age, which together with high-resolution paleomagnetic secular variations allow synchronization with a well-dated marine core from the shelf north of Iceland. A composite standardized climate record from the two lakes provides a single time series that efficiently integrates multi-proxy data that reflect the evolution of summer temperatures through the Holocene. The first-order trends in biogenic silica (BSi), d13C, and C:N rise relatively abruptly following deglaciation, reaching maximum values shortly after 8 ka following a complex minimum between 8.7 and 8.0 ka. The Holocene Thermal Maximum (HTM) in the lakes is marked by all proxies, with a sharp transition out of the 8 ka cold event into peak summer warmth by 7.9 ka, and continuing warm with some fluctuations until 5.5 ka. Decreasing summer insolation after the HTM is reflected by incremental cooling, initially ~5.5 ka, with subsequent cold perturbations recorded by all proxies 4.3 to 4.0 ka and 3.1 to 2.8 ka. The strongest disturbance occurred after 2 ka with initial summer cooling occurring between 1.4 and 1.0 ka, followed by a more severe drop in summer temperatures after 0.7 ka culminating between 0.5 and 0.2 ka. Following each late Holocene cold departure, BSi re-equilibrated at a lower value independent of the sediment accumulation rate. Some of the abrupt shifts may be related to Icelandic volcanism influencing catchment stability, but the lack of a full recovery to pre-existing values after the perturbation suggests increased periglacial activity, decreased vegetation cover, and glacier growth in the highlands of Iceland. The similarity in timing, direction and magnitude of our multi-proxy records from glacial and non-glacial lakes, and from the adjacent marine shelf, suggests that our composite record reflects large-scale shifts in ocean/atmosphere circulation throughout the northern North Atlantic.

两个沉积积累速率较高的冰岛湖泊——冰川湖赫伊塔瓦特恩（Hvítárvatn）与非冰川湖豪克达尔瓦特恩（Haukadalsvatn），保存了大量定年明确的火山碎屑层（tephra layers）；结合高分辨率古地磁长期变化（paleomagnetic secular variations）数据，可与冰岛北部陆架上定年完善的深海岩芯开展年代同步。基于这两个湖泊构建的标准化综合气候记录，形成一条统一的时间序列，可高效整合反映全新世（Holocene）夏季温度演化的多代用指标数据。生物成因二氧化硅（biogenic silica, BSi）、δ¹³C以及C:N比值的一级变化趋势在冰消期（deglaciation）后相对快速抬升，在8.7~8.0 ka期间经历复杂的低值阶段后，于8 ka之后不久达到峰值。所有代用指标均显示，湖泊记录中的全新世大暖期（Holocene Thermal Maximum, HTM）以从8 ka冷事件快速转型为特征：至7.9 ka达到夏季温度峰值，后续气候持续偏暖并伴随小幅波动，直至5.5 ka。全新世大暖期结束后，夏季太阳辐射的减弱体现在温度逐步降低，初始降温始于约5.5 ka；随后在4.3~4.0 ka与3.1~2.8 ka期间，所有代用指标均记录到寒冷扰动事件。最为显著的扰动发生在2 ka之后：首次夏季降温出现于1.4~1.0 ka区间，随后在0.7 ka之后夏季温度出现更为剧烈的下降，并在0.5~0.2 ka期间达到低谷。在每次全新世晚期的寒冷事件结束后，生物成因二氧化硅均会在较低水平重新达到平衡，且该过程与沉积积累速率无关。部分突变事件可能与冰岛火山活动影响流域稳定性有关，但扰动后生态系统未能完全恢复至原有状态，这表明冰岛高地的冰缘活动增强、植被覆盖度下降，同时冰川出现扩张。我们基于冰川湖、非冰川湖以及邻近陆架的多代用指标记录，在事件发生时间、变化方向与幅度上均具有高度相似性，这表明该综合气候记录反映了整个北大西洋北部海域-大气环流的大规模变迁。