Stable oxygen isotopes and winter bottom water temperature data from Gullmar Fjord, Skagerrak (North Sea)@en

We present 2500 years of reconstructed bottom-water temperatures (BWT) in annual to subdecadal resolution by using a fjord sediment archive from the NE Atlantic region. The BWT represent winter conditions due to the fjord hydrography and associated timing and frequency of bottom-water renewals. The study is based on a ca. 8-m long sediment core from Gullmar Fjord (Sweden), dated by 210Pb and AMS 14C and analysed for stable oxygen isotopes (δ18O) measured on shallow infaunal benthic foraminiferal species Cassidulina laevigata. The BWT, calculated by using the palaeotemperature equation of McCorkle et al (1997), range between 2.7 - 7.8°C and are within the annual temperature variability, instrumentally recorded in the deep fjord basin since the 1890s. The record demonstrates a warming during the Roman Warm Period (~350 BCE - 450 CE), variable BWT during the Dark Ages (~450 - 850 CE), positive BWT anomalies during the Viking Age/Medieval Climate Anomaly (~850 - 1350 CE) and a long-term cooling with distinct multidecadal variability during the Little Ice Age (~1350 - 1850 CE). The fjord BWT record also picks up the contemporary warming of the 20th century (presented here until 1996), which does not stand out in the 2500-year perspective and is of the same magnitude as the Roman Warm Period and the Medieval Climate Anomaly.本研究基于东北大西洋区域的峡湾沉积档案，重建了2500年以来年际至亚年代际分辨率的底层水温（bottom-water temperatures, BWT）序列。受峡湾水文特征以及底层水更新的时间与频次影响，该BWT序列反映的为冬季底层水温状况。本研究的核心数据取自瑞典古尔马尔峡湾（Gullmar Fjord）一根长约8米的沉积岩芯，通过210Pb与加速器质谱碳-14（AMS 14C）测年，并对采自该岩芯的浅栖底栖有孔虫物种光亮抱环虫（Cassidulina laevigata）开展了稳定氧同位素（δ18O）分析。基于McCorkle等人1997年的古温度计算公式重建的BWT序列，其数值范围为2.7~7.8℃，与19世纪90年代以来在峡湾深水区仪器观测得到的年际温度变幅范围一致。该序列显示，罗马暖期（约公元前350年—公元450年）存在显著升温；黑暗时代（约公元450年—850年）底层水温波动剧烈；维京时代/中世纪暖期（约公元850年—1350年）出现正BWT异常；小冰期（约公元1350年—1850年）则呈现长期降温趋势，且伴随显著的年代际尺度变率。该峡湾BWT序列还记录了20世纪的当代升温过程（本次研究序列截至1996年），但从2500年的时间尺度来看，此次升温并不突出，其幅度与罗马暖期及中世纪暖期相当。