2018, Vol.38
北大西洋作为水-热-气交互作用强烈的地区，在全球气候系统中占据至关重要的地位。大西洋经向翻转环流(Atlantic Meridional Overturning Circulation，简称AMOC)将低纬地区温暖的表层水传送到高纬，并在高纬海域向大气释放热量，从而导致这些海水冷却、下沉、并向南返程传输[1~2]。近年来一系列观测研究结果发现，格陵兰冰盖的消融正在加速，北大西洋海水盐度呈现出显著的淡化趋势，一些模式的结果显示在全球变暖的情形下，大量淡水注入北大西洋高纬海域将降低表层海水的密度和深海对流，AMOC的强度会急剧减弱甚至崩溃，这将对北大西洋乃至全球气候产生重要的影响[3~5]。
深海氧同位素(Marine Isotope Stage，简称MIS)5e是地质历史中距今最近的一次间冰期(即末次间冰期[6])，其气候环境可与现代暖期(即全新世)对比[7]。已有的研究表明MIS 5e的全球平均海表温度比工业革命前高约0.5 ℃，且高纬地区的暖化更加明显[8~9]。尽管存在时空差异，地质记录揭示MIS 5e全球海陆地表平均温度比工业革命前高约1 ℃[9]，海平面最高比现在高6~9 m[10]。对比没有人类工业活动影响的MIS 5e时期和全新世的气候变化，有助于更好地理解自然变率下的暖期过程和未来气候发展趋势，以及全球变暖对其他气候要素(比如AMOC)的影响。
深海沉积物中有孔虫的氧同位素是古海洋古气候研究中应用最广泛的地球化学指标之一，可以用来重建海水温度[11~16]和全球大陆冰量的变化[17~18]。有孔虫的氧同位素组成主要取决于有孔虫壳体钙化时周围海水的氧同位素组成和影响氧同位素分馏的海水温度[11, 17, 19]。对于浮游有孔虫来说，不同种属的钙化深度和季节不尽相同[19]；而对于底栖有孔虫来说，这个深度通常是海洋沉积物的深度。由于受全球和区域气候环境的影响，海水的氧同位素组成会随时空变化[20~21]。全球性的影响主要是冰量效应[17]，比如在末次冰盛期(Last Glacial Maximum，简称LGM)，由于大规模的冰盖扩张，全球海水平均氧同位素组成比现在重约1 ‰ [20]。除此之外，区域海水性质变化也会影响当地海水的氧同位素组成，比如海水温度、来自不同源区具有不同氧同位素信号的水团混合，或局部区域内的蒸发、降水、海冰的形成与消融[21]，其中温度效应是最重要因素[11, 22]。在平衡分馏的情况下，温度每升高1 ℃，碳酸钙壳体的氧同位素变轻约0.20 ‰~0.25 ‰ [22~23]。总的来说，底栖有孔虫氧同位素记录部分反映了全球冰量的消长，其波动特征反映了冰期与间冰期交替的频度和幅度[18]，常作为深海沉积物的定年标尺[24]。目前，基于底栖有孔虫氧同位素的全球和区域性的平均化时间序列已建立，但区域内系统的氧同位素演化对比研究仍较少。因此，本文拟通过整合北大西洋深海沉积物钻孔全新世和MIS 5e底栖有孔虫氧同位素记录，综合分析该区古海洋变化，为理解现代暖期过程和未来气候变化趋势提供依据。
1 研究方法本文对北大西洋已发表的全新世和MIS 5e时期的深水底栖有孔虫氧同位素记录进行了汇总和统计。数据选取原则如下：1)钻孔水深超过1 km；2)钻孔具有明确且连续的时间序列；3)氧同位素数据点在全新世(0~12 ka)和MIS 5e(116~130 ka)[6]两个阶段的时间分辨率都要超过3 ka。据此，本文共筛选出了47个沉积物钻孔(图 1)的底栖有孔虫氧同位素序列，并根据深度和地理位置分为4个区域：1~2 km深度的①北大西洋中层水(Intermediate North Atlantic Water，简称INA)，2 km以深的②高纬北大西洋深水(45°N以北；Northern Deep North Atlantic water，简称N. DNA)，③中低纬西北大西洋深水(45°N以南、洋中脊以西；Western Deep North Atlantic Water，简称W. DNA)和④中低纬东北大西洋深水(45°N以南、洋中脊以东；Eastern Deep North Atlantic Water，简称E. DNA)。
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图 1 北大西洋47个钻孔的位置图 钻孔颜色代表4个不同区域(与图 3和4一致)：绿色—①北大西洋中层水(INA)；蓝色—②高纬北大西洋深水(N. DNA)；橙色—③中低纬西北大西洋深水(W. DNA)；黄色—④中低纬东北大西洋深水(E. DNA) Fig. 1 The locations of the 47 sediment cores in the North Atlantic. The cores are divided into four regions by symbols in different colours:①INA(green), ②N. DNA(blue), ③W. DNA(orange), ④E. DNA(yellow). Same colour coding in the following Fig. 3 and Fig. 4 |
由于本文统计的氧同位素数据来自全世界各个不同的实验室，在比较数据之前，有必要对数据误差进行分析。有孔虫碳氧同位素测试的常规方法是磷酸法，即在真空环境下让清洗过的有孔虫粉末(与标样)和100 %正磷酸恒温反应，析出CO2供质谱分析，同时测量样品和标样的同位素值，最后样品报道值都将通过标样的真实值来校正。对于氧同位素而言，不同实验室(或者不同时期)采用的反应温度不尽相同(通常为50 ℃、70 ℃、75 ℃或90 ℃)，但都保证了标样和样品的测试是控制在同一个温度下进行的，因此氧同位素的真实值即可通过特定温度对应的酸分馏系数校正。尽管不同实验室可能采用不同的反应温度或者不同的进样系统和测试仪器，将样品报道值统一校正到统一的国际标准(比如标样NBS19)，保障了不同实验室之间数据对比的可靠性[25]。通常实验室报道的测试误差，即各个实验室在测具体批次样品期间(或包含这个时间段更长的一个时期，比如1年内)反复测试的标样的标准偏差，包括了所有实验过程和仪器测试过程中产生的误差。本文中所有实验室报道的δ18O测试误差(± 1σ)都小于0.1 ‰ (表 1)，但实际上制约同位素值比较的并不是测试误差，而是不同有孔虫壳体同位素值的可重复性。已有的研究发现同一层位海洋沉积物同一粒径范围内挑出的同一批Cibicidoides wuellerstorfi在不同实验室[26]、同一实验室不同仪器[27]和同一实验室同一仪器[28]的重复测试标准偏差(± 1σ)为0.10 ‰~0.18 ‰。
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表 1 北大西洋47个深海沉积物钻孔位置与全新世和MIS 5e氧同位素数据* Table 1 Locations of the 47 sediment cores used in this study and their Holocene and MIS 5e oxygen isotopic values |
Cibicidoides spp.是北大西洋最常见的底栖有孔虫种，其中Cibicidoides wuellerstorfi是最常用的属。因此，本文绝大部分氧同位素的测试都基于这一种(属)，只有2个钻孔的测试基于其他种属(Unigerina spp.)。考虑到以往全球和区域性氧同位素平均化曲线(包括早年报道的数据表)都是将氧同位素校正到Unigerina标准上[24]，本文仍根据古海洋古气候领域最常用的转换方法将氧同位素数据都校正到Unigerina标准(Unigerina=Cibicidoides spp.+0.64)[18]。尽管该校正存在不确定性和统计误差[22]，对于本文所统计的47个钻孔数据而言，因为会受校正影响的钻孔至多2个，预计对本文的结论影响不大。
为了提高全新世和MIS 5e氧同位素对比的准确性，本文根据每个钻孔氧同位素分布和演化特征(图 2)，选取了这两个间冰期时间窗口内(12~0 ka和130~116 ka)具有相同时间跨度的稳定期来对比，同时采用了两种方法来验证对比的可靠性：平均值法和极低值法(表 1)。对于气候环境差别不大的间冰期而言，氧同位素的变化往往在误差范围内，因此本文尝试通过对比北大西洋大量钻孔全新世和MIS 5e的氧同位素，检验数据是否存在系统的变化。即使数据变化小而误差大，只要变化具有规律性，从统计的角度依然是可以分辨气候变化导致的潜在氧同位素变化。
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图 2 北大西洋中层水(INA)全新世(a)和MIS 5e (b)、高纬北大西洋深水(N. DNA)全新世(c)和MIS 5e (d)、中低纬西北大西洋深水(W. DNA)全新世(e)和MIS 5e (f)以及中低纬东北大西洋深水(E. DNA)全新世(g)和MIS 5e (h)的底栖有孔虫氧同位素序列对比 氧同位素数据分析误差1σ=0.1 ‰，如图左下角所示 Fig. 2 The Holocene(left column)and MIS 5e(right column)benthic δ18O records(corrected to Uvigerina peregrina) in the four regions:(a, b)INA; (c, d)N. DNA; (e, f)W. DNA; (g, h)E. DNA. Analytical errors(1σ=0.1 ‰) are shown on the left corner of each panel |
全新世和MIS 5e底栖有孔虫氧同位素演化趋势的对比显示，全新世的氧同位素比值在约8~10 ka之后较为稳定；而即使考虑了定年精度和MIS 5e起止时间在不同钻孔略有不同[7]，MIS 5e的氧同位素演化(图 2b、2d、2f和2h)总体来说不如全新世(图 2a、2c、2e和2 g)稳定，且高纬深水(图 2d)稳定性高于其他区域(图 2b、2f和2h)。相对稳定的全新世氧同位素演化反映了本文所统计的数据的可重复性较好，因此采纳最大的测试误差(即0.1 ‰)作为本文数据的分析误差。
表 1中全新世和MIS 5e的统计结果显示：全新世氧同位素平均值为2.77 ‰~3.95 ‰，MIS 5e为2.72 ‰~3.77 ‰，全新世与MIS 5e差值为- 0.58 ‰~0.41 ‰，其中28个钻孔差值大于误差(0.1 ‰)；全新世氧同位素极低值为2.37 ‰~3.85 ‰，MIS 5e为2.46 ‰~3.68 ‰，全新世与MIS 5e差值为- 0.60 ‰~0.49 ‰，其中33个钻孔差值大于误差(0.1 ‰)。受温度效应影响，氧同位素的分布呈现明显的地域差异，高纬(特别是亚极地)地区的钻孔氧同位素值最高(高达约4.0±0.1 ‰)，氧同位素低值(低至约2.8±0.1 ‰)都见于水深小于约1.5 km的钻孔(表 1和图 3)，可见氧同位素的系统变化是可以通过区域内的对比识别的。同时满足平均值和极低值变化大于0.1 ‰的钻孔约占一半(有24个)，说明这两个间冰期存在系统的氧同位素变化。T-检验(T-test)结果也证明尽管差别较小，在95 %的置信区间内，47个钻孔全新世的平均值和极低值都显著大于MIS 5e的平均值和极低值，进一步对4个区域分别做T-test显示，其中高纬北大西洋深水(N. DNA)和中低纬西北大西洋深水(W. DNA)全新世的平均值和极低值均显著大于MIS 5e，而其他两个区域无系统差别。两种方法的结果相吻合也证明了氧同位素对比方法的可靠性。为了减小对比数据的随机性和分布不同时造成的不确定性，本文关于全新世和MIS 5e氧同位素差异的讨论将主要基于平均值法(图 3和图 4)。
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图 3 北大西洋中层水(a)、高纬北大西洋深水(b)、中低纬西北大西洋深水(c)和中低纬东北大西洋深水(d)钻孔底栖有孔虫全新世(实心)和MIS 5e(空心)氧同位素平均值随深度的分布 氧同位素数据分析误差1σ=0.1 ‰，如图上方所示 Fig. 3 Depth distribution of averaged Holocene(filled symbols)and MIS 5e(open symbols)benthic δ18O in (a)INA, (b)N. DNA, (c)W.DNA, (d)E.DNA. Analytical errors(1σ=0.1 ‰) are shown on the top of each panel |
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图 4 北大西洋中层水(a)、高纬北大西洋深水(b)、中低纬西北大西洋深水(c)和中低纬东北大西洋深水(d)钻孔底栖有孔虫氧同位素平均值在全新世和MIS 5e的对比 氧同位素数据分析误差1σ=0.1 ‰，如图右下角所示 Fig. 4 Comparison between the averaged Holocene and MIS 5e benthic δ18O in (a) INA, (b)N. DNA, (c)W. DNA, (d)E. DNA. Analytical errors(1σ=0.1 ‰)are shown on the right corner of each panel |
尽管北大西洋区域内MIS 5e底栖有孔虫氧同位素系统性的比全新世轻，仍有两个地区MIS 5e的氧同位素值明显比全新世重：一是位于东部北大西洋中层水(INA)深度不超过约1.5 km的钻孔ODP982和地中海附近的GIK16004 - 1与GeoB4240-2，偏重约0.25 ‰~0.58±0.08 ‰ (表 1、图 3a和4 a)；二是位于赤道地区的东北大西洋深水(E. DNA)2~3 km内的钻孔GIK13519-1和GeoB4901-8，偏重约0.1 ‰~0.2±0.1 ‰ (表 1、图 3d和4d)。
3 讨论如果不考虑冰盖消融的全球效应，盐度对远离大陆边缘、受蒸发和降水影响较小的深水(> 2 km)影响很小[21]。因此，温度和冰量是控制底栖有孔虫氧同位素最重要的两个因素。从本文统计的结果来看，北大西洋深海沉积物MIS 5e的氧同位素比全新世系统性地偏轻，反映MIS 5e的深海温度高于全新世或更多相对富16O的冰川消融。尽管全球不同地区重建的MIS 5e海平面高度不尽相同，如早年基于太平洋和印度洋珊瑚礁U/Th测年数据认为MIS 5e海平面最高比现在高4~6 m[72~73]，近年来更系统的海平面重建工作认为MIS 5e全球平均海平面比现在高6~9 m[74~75]。总的来说，目前的共识是MIS 5e海平面比全新世高。为定量化海平面变化对氧同位素的响应，本文的工作假设是MIS 5e海平面比全新世高6 m，富16 O冰盖的消融使得海平面每升高1 m，海水氧同位素平均变轻0.01 ‰ [76]，对应MIS 5e海水氧同位素比全新世轻约0.06 ‰。需要说明的是，本文采用的氧同位素与海平面变化的换算系数是根据冰期-间冰期尺度的估算[20, 77~78]。MIS 5e氧同位素与海平面变化的关系还缺少冰盖δ18O的可靠估计，所以存在一定的不确定性。而东赤道太平洋科科斯海岭(Cocos Ridge)浮游有孔虫δ18O-Mg/Ca数据表明在MIS 5e这一系数仍然适用[76]，因此本文还是沿用了该系数。对于MIS 5e底栖有孔虫氧同位素平均值和极低值都比全新世轻> 0.1 ‰的钻孔(平均轻0.21 ‰)而言，仅仅海平面变高不足以解释0.21 ‰这么大的差异。深水温度和水团性质变化应在不同程度上对北大西洋MIS 5e的轻δ18O有贡献。
关于重建MIS 5e北大西洋深水温度的研究较少。目前，深水温度的重建主要依靠底栖有孔虫和介形虫的Mg/Ca。从已有的现代钻孔顶部沉积物底栖有孔虫或介形虫Mg/Ca与实测深水温度的关系来看，在1~4 ℃范围内Mg/Ca变化幅度大且相关性差[79~80]，重建误差甚至大于实际温差变化幅度；而且当海水碳酸盐离子浓度与饱和浓度之差(ΔCO32-)小于20 μmol/kg时，C.wuellerstorfi的Mg/Ca比值会随ΔCO32-的降低显著下降[81~82]，从而影响对温度变化重建的可靠性。已有的研究指出MIS 5e的深水温度与全新世非常接近[12, 80, 83]，但赤道东大西洋MIS 5e的峰值温度比全新世略低[80]，可能导致该地区钻孔(GIK13519-1、GIK13521-1、GeoB1101-5和GeoB4901-8)底栖有孔虫MIS 5e与全新世的氧同位素差值更小甚至为负(表 1)。基于中低纬西北大西洋深水(W. DNA)的钻孔CHN82-24介形虫壳体Mg/Ca的古海水温度重建指出MIS 5e深水温度(约1.3 ℃)比全新世(约3.2 ℃)低约2 ℃[79, 84]。根据最新的全球范围内底栖有孔虫Cibicidoides氧同位素与海水分馏的温度效应的校准工作(calibration)[22]所揭示的两者对应系数约为- 0.25±0.02 ‰ /℃，即若深海降低2 ℃，对应氧同位素重约0.5±0.04 ‰。而该钻孔MIS 5e底栖有孔虫氧同位素平均值比全新世轻0.02 ‰，极低值比全新世轻0.19 ‰ (表 1)，即使考虑海平面升高6 m引起的- 0.06 ‰，仍无法产生高达2 ℃的温度变化幅度。本文认为如果只考虑冰量和温度对氧同位素的影响，该地区MIS 5e至多比全新世低约0.2 ℃，因此低温环境下Mg/Ca温度计的可靠性仍有待进一步的研究。由于现代2 km以浅的海水温度(60°N以北冰岛附近的ODP983和ODP984除外)显著高于之下的深水，所以中层水(1~2 km)有孔虫氧同位素组成较深水的偏低(见表 1和图 3，分别为3 ‰和3.5 ‰左右)。北大西洋深水(> 2 km)MIS 5e有孔虫氧同位素平均值和极低值均比全新世平均轻0.1 ‰，如果假设MIS 5e海平面比全新世高6 m(- 0.06 ‰)，那么北大西洋深水温度可能比全新世高约0.2 ℃，这与西南太平洋MIS 5e和全新世的深水温差0.19 ℃相近[85]。类似地，本文发现MIS 5e时期高纬北大西洋深水(N. DNA)比中低纬地区(W. DNA和E. DNA)变暖更为显著，平均温度比全新世分别升高了0.2 ℃和0.1 ℃，这与区域性海表温度重建和模拟结果[9]相吻合。
在北大西洋47个钻孔中，位于挪威海2525 m水深的钻孔V27-60，现在水温低至- 1.05 ℃，在全新世和MIS 5e都有着所有钻孔中最高的氧同位素值(表 1)，可能反映其温度在MIS 5e也最低。钻孔V27-60的MIS 5e氧同位素比全新世轻0.18 ‰，海平面变化可以解释其中的0.06 ‰，剩余的0.12 ‰可能原因是：1)海水温度升高约0.5 ℃；2)海水氧同位素组成本身变轻。挪威海常年被冰覆盖导致水深超过1 km处温度低至- 1±0.2 ℃，在这种情况下深水温度不太可能发生变化，因此挪威海深水沉积物底栖有孔虫氧同位素能够记录海水氧同位素变化[12]。MIS 5e挪威海氧同位素变轻可能是由淡水输入增多所致[13]，贫18 O的淡水可能源自海表高温引起的高蒸发和高纬的高降水(北冰洋和周围大陆)[86]，或由于海平面升高导致北太平洋海水通过白令海峡输入挪威海的量变大[87]，或相对温暖的表层水下沉形成深水的强度增强[88]。
洋流变化导致钻孔所记录的水团信号发生改变也可以影响氧同位素组成[21]。北大西洋深水(North Atlantic Deep Water，简称NADW)是研究区最重要的水团，也是驱动现代大洋温盐环流系统最主要的水团[1]。NADW主要形成在北极高纬地区，温暖的表层洋流在向北传输的过程中通过蒸发和与其他水团混合导致盐度增加，当到达北大西洋时由于温度降低、盐度增加而下沉，并向南半球传输[1, 89]。北大西洋深水(NADW)可以分成上部深水(upper NADW，< 2 km)和下部深水(lower NADW，> 2 km)[89]。其中，上部深水由该地区不同的中层水组成，包括拉布拉多海水(Labrador Sea Water，简称LSW)、地中海溢出流(Mediterranean Overflow Water，简称MOW)和冰岛-苏格兰海脊溢出流(Iceland Scotland Overflow Water，简称ISOW)；而下部深水由来自北欧海的高密度丹麦海峡溢出流(Denmark Strait Overflow Water，简称DSOW)和LSW混合而成。
北大西洋中层水(1~2 km)的MIS 5e和全新世氧同位素的差值最大(表 1)，1.5 km以上的3个钻孔(ODP982、GIK16004-1和GeoB4240-2)MIS 5e氧同位素值比全新世重0.25 ‰~0.58±0.08 ‰；1.5~2.0 km之间的4个钻孔(高纬ODP983、ODP984和低纬GeoB3935、GeoB3938)MIS 5e氧同位素值则比全新世轻0.12 ‰~0.24±0.1 ‰。考虑到海平面变化对海水氧同位素的影响，前者MIS 5e将比全新世重0.31 ‰~0.64 ‰，而后者比全新世轻0.06 ‰~0.18 ‰。已有的研究表明MIS 5e北大西洋深水环流强度和化学性质与现在差别不大[56, 90~92]，那么1.5~2.0 km的中层水有孔虫氧同位素的变化可能是由于MIS 5e温度比全新世高0.2~0.7 ℃所致。
温暖高盐的MOW是影响北大西洋1.5 km以上海水的主要水团之一，它从地中海通过直布罗陀(Gibraltar)海峡沿着欧洲西海岸向高纬传输，同时向西注入大西洋，它是大西洋温跃层(thermocline)以下高温和高盐水惟一的来源[89, 93]。图 3a中，1.5 km以浅的钻孔MIS 5e有孔虫氧同位素值比全新世重0.25 ‰~0.58 ‰，如果仅用温度效应来解释，那么MIS 5e温度需要比全新世低1.2~2.6 ℃。因为没有相关的重建温度，尽管存在1~3 ℃降温的可能性，考虑到MIS 5e地中海海表温度比现在高3~5 ℃[94~95]，本文认为MOW形成减弱，向大西洋输出减少，导致高纬低温中层水在1.5 km之上的上部海洋中的比例增加可能是上层海洋有孔虫氧同位素比值在MIS 5e显著升高的主要原因。MIS 5e时期MOW的减弱得到了来自地中海的沉积物粒度和底栖有孔虫碳同位素和有孔虫组合证据的支持[96~97]。与南海北部淡水收支主要受夏季风降水的影响类似[98]，由于非洲季风加强导致注入地中海的淡水量增加，加强了地中海垂向分层，使得深水缺氧而形成广泛的腐泥质层S5[99~100]，这种分层又抑制了中层水的下沉和向大西洋的输出[101]。Bahr等[101]又进一步指出MOW减弱的驱动因子是增强的北半球夏季太阳辐射，而导致的结果是影响了大西洋的盐度平衡和AMOC的稳定性[102]，使得MIS 5e氧同位素的演化没有全新世稳定(图 2)。气候模拟显示全球变暖将限制NADW在拉布拉多海盆下沉和环流，而只能在格林兰岛-冰岛-苏格兰岛海脊以北形成[103]。这一结果得到了地质记录的支持，水团同位素示踪[91]指出MIS 5e时期NADW的形成以东北大西洋北欧海为主，同时拉布拉多海盆内LSW和NADW的分层缺失[104]，反映这一时期拉布拉多海盆内没有深水形成。总的来说，MIS 5e时期北大西洋1 km以深的海水表现出整体比全新世更暖的趋势，可能与更高的北半球夏季太阳辐射量[105~106]相关。全球变暖导致水循环增强——高纬冰川消融，地中海降水量变大，使得北大西洋表层海水盐度和密度降低，弱化了NADW的生成而导致AMOC减弱[102]。正是这种暖化背景下高纬淡水通量的增加，使得MIS 5e在NADW形成率和AMOC强度与全新世差不多的情况下，气候不如全新世稳定，在高分辨率的样品中甚至出现了百年尺度的NADW减弱和降温事件[95, 107~108]。现代观测也发现在如今全球变暖的趋势下，北大西洋持续的暖化和AMOC加强[109]自90年代后有所减弱[110]，从2005年开始北大西洋上层海洋温度甚至下降了约0.45 ℃[111]，以至于AMOC出现减弱[3]。气候模型也指出在未来气候变暖的环境下，MIS 5e时期出现的淡水通量强迫幅度增加导致AMOC在不同模式中波动的现象很有可能在未来重现[102]。由于目前已有的深海沉积物记录的时间分辨率不足和定年误差较大，本文无法深入探讨MIS 5e百年尺度的气候事件。寻找高分辨率的古环境记录，识别气候突变事件，优化环境模型，是古气候古环境研究从现象描述向极力探索的转变，是未来古气候学家研究的当务之急[112~113]。
4 结论即使按照IPCC第五次评估报告中最乐观的碳排放估计，即典型浓度路径2.6(RCP2.6)的情况下，全球平均表面温度也将在本世纪中叶达到比工业活动前升高1 ℃[114]。而MIS 5e全球平均温度比全新世高约1 ℃，将是未来气候和环境变化的真实写照。了解MIS 5e的气候要素，可以为我们预测未来海气系统的变化趋势提供约束。本文尝试通过对比区域内MIS 5e和全新世时期的氧同位素数据，来探究最近两个间冰期的差异是否能从氧同位素记录中识别。通过对北大西洋47个钻孔深海沉积物底栖有孔虫氧同位素结果的统计，发现了MIS 5e氧同位素平均值和极低值比全新世系统性偏轻的趋势，反映了海平面升高对整个区域的影响。当扣除海平面升高对氧同位素的影响后，MIS 5e仍比全新世氧同位素更轻，指示了北大西洋深水温度整体偏高，且高纬地区深水平均温度的升高比中低纬地区更为显著。1.0~1.5 km的中层水有孔虫氧同位素在MIS 5e的显著偏低，可能与注入地中海的淡水量增加减弱了MOW的形成率和向大西洋的输出有关。因此，本文的认识将有助于进一步约束北大西洋对气候变暖响应的机制研究。
致谢: 非常感谢评审专家提出的宝贵意见和杨美芳老师、鹿化煜老师对文章修改的大力帮助，在此表示感谢。
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Abstract
The North Atlantic plays an important role in the global climate system and its link with climate change has received much attention. Marine Isotope Stage(MIS) 5e with a warmer climate than the Holocene is considered as an analogy for future warming climate. To date, there are few deepwater temperature and salinity comparisons between these two recent interglacial periods in the North Atlantic. Benthic foraminifera oxygen isotope(δ18O) has long been analysed in paleoclimate studies since its variation closely related to global ice volume, temperature and ocean circulation changes. In this study, a comparison of benthic foraminifera oxygen isotope during the Holocene(0~12 ka) and MIS 5e(116~130 ka) stages from 47 deep-sea sediment cores in the North Atlantic is presented to investigate whether a systematic δ18O difference exists during the last two interglacial periods. This database suggests the δ18O signatures during the MIS 5e were systematically lighter(ca. 0.08 ‰) than those in the Holocene, indicating potential reduced global continent ice volume and/or warmer deep-sea temperature in the North Atlantic. Although the δ18O differences between Holocene and MIS 5e are small, the systematical trends in our database confirmed by t-test provide confidence in our proposal that the different climate signals are observed by the δ18O comparison. This study further examines the contribution of sea level or temperature changes in the MIS 5e and Holocene δ18O differences tentatively based on the empirical relationships between δ18O and sea level/temperature in the palaeoceanography field, which suggests a dominant control of higher sea level and a slight warmer North Atlantic deepwater in the MIS 5e. A further regional examination of δ18O difference between Holocene and MIS 5e shows larger temporal variations in the intermediate depth cores between 1 and 2 km water depths (> 0.2 ‰) than cores below, implying enlarged magnitude of temperature changes, and the heavier δ18O of cores above 1.5 km during the MIS 5e averaged by ca.0.36 ‰ might be associated with regional ocean circulation changes. Moreover, the decrease of ca.0.12 ‰ in MIS 5e benthic foraminifera δ18O of high latitude deep cores(> 45°N) is more pronounced than that of low-middle latitude deep cores(0°~45°N), inferring a more significant warming in the high latitude deep water, which is in accordance with sea surface temperature reconstructions and modelling results. Benthic oxygen isotope investigation based on multiple sediment cores is a useful approach to identify regional changes in oceanographic changes. Changes in regions such as the high latitude and intermediate depth North Atlantic would deserve special attention in future climate simulations.