Comment on “Abrupt change in tropical African climate linked to the bipolar seesaw over the past 55,000 years” by E. T. Brown, T. C. Johnson, C. A. Scholz, A. S. Cohen, and J. W. King
Comment on “Abrupt change in tropical African climate linked to the bipolar seesaw over the past 55,000 years” by E. T. Brown, T. C. Johnson, C. A. Scholz, A. S. Cohen, and J. W. King
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E. T. Brown、T. C. Johnson、C. A. Scholz、A. S. Cohen 和 J. W. King 对“热带非洲气候的突变与过去 55,000 年的两极跷跷板有关”的评论
DOI：
10.1029/2007gl032399
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发表时间：
2008
影响因子：
5.2
通讯作者：
Y. Garcin
中科院分区：
文献类型：
--
作者：
Y. Garcin
[1] Brown等[2007]（以下简称BJSCK07）利用XRF扫描仪测量湖泊沉积物中元素Zr:Ti的比值，重建了马拉维湖陆源输入的历史。由此产生的记录是非洲热带大陆首次明确显示百年至千年气候变化，即所谓的Dansgaard-Oeschger (D-O)，通常在末次冰期的北半球高纬度地区发现。引人注目的是，BJSCK07对马拉维湖记录的看法表明，热带非洲的气候突变导致了格陵兰岛大约1200年的类似气候突变，并与热两极跷跷板机制相结合。然而，在这里，我提出另一种古环境解释可能会为这一古气候记录提供一个新的视角，不那么投机。[2]马拉维湖记录中的Zr:Ti峰值与NGRIP dO记录中的D-O间冰期（变暖）具有明显的良好相关性，这与Brown等[2006]先前的假设一致。同样，马拉维湖Zr:Ti记录的主要趋势似乎与NGRIP dO记录的非常相似。然而，BJSCK07将马拉维湖记录中的Zr:Ti峰值与D-O运动和Heinrich事件（冷却）相关联。[3]当BJSCK07试图将马拉维湖记录中的Zr:Ti峰与D-O间岩相匹配时，遇到的最大问题之一是他们之前对马拉维湖沉积物中Nb:Ti比例的解释（模拟Zr:Ti比例：铌和锆是不相容的元素，它们在火山岩中很常见，与更具流动性的钛元素相比，它们在火山玻璃基质中相对丰富）。Brown和Johnson[2005]以及Johnson等人[2002]先前对Nb:Ti峰值进行了解释，认为北风吹过Rungwe地区（马拉维湖集水区北部）凉爽干燥的火山地貌，将火山灰输送到湖泊，导致湖泊沉积物中Nb:Ti含量较高。根据BJSCK07，这种情景在机械上与D-O间冰期对应的气候条件不一致，因为在这些“温暖”事件期间，热带辐合带（ITCZ）可能位于北部，因此马拉维湖盆地的大气环流以东南风为主。[4] Johnson等人[2002]先前在新仙女木事件（YD）期间在马拉维湖沉积物中发现了一个强烈的Nb:Ti峰值，随后声称这是一个干燥的气候时期，在此期间，ITCZ位于马拉维湖以南，区域大气环流以强烈的北风为主。然而，最近对位于Rungwe地区中心的Masoko湖沉积物核的花粉研究表明，在过去的45,000年中，这个火山省最发达的树木覆盖发生在YD时期[Garcin et al., 2006]。这表明来自Rungwe地区的风蚀作用在YD期间减少或不存在，而Rungwe地区被认为是马拉维湖铌和锆的主要来源[Garcin et al., 2006]。此外，在YD期间，在Masoko湖沉积物中观察到几个厘米厚的碲层[Garcin et al., 2006]。这些富含铌的暖层（Y. Garcin和D. Williamson，未发表的数据，2006年）也可能是在YD期间马拉维湖沉积物中记录到Nb:Ti峰值的原因之一。然而，在马拉维湖沉积物岩心中没有观察到这些离散的暖层[Johnson等人，2002]，因为马拉维湖的岩心位置远于主要的Rungwe火山中心（约80-100公里）。总之，BJSCK07将复杂的YD期定义为解释55000年记录的参考期，可能受到Rungwe火山活动的过度污染，无法推断马拉维湖沉积物中Nb:Ti和Zr:Ti的“正常”行为。[7] BJSCK07仅根据风的变化解释了Nb:Ti和Zr:Ti比值的波动，他们参考了Brown and Johnson[2005]和Johnson et al.[2002]对其解释的更多细节。然而，在这两篇文献中都没有对集水区岩石、土壤和河流沉积物负荷进行化学分析，以支持沉积物岩心中Nb:Ti和Zr:Ti比值的解释。[10]马拉维湖盆地北部为非均质盆地，由二叠统-三叠纪（Karoo）至新近纪的沉积物和火山覆盖的前寒武纪基底组成[Ebinger et al., 1993]。这些岩石可能具有截然不同的铌、锆和钛特征[j] .物理研究快报，VOL. 35, L04701, doi:10.1029/2007GL032399, 2008
[1] Brown et al. [2007] (hereinafter referred to as BJSCK07) have reconstructed the history of terrigenous inputs into the Lake Malawi by measuring the elemental Zr:Ti ratio on lake sediments with an XRF scanner. The resulting record is the first in continental tropical Africa that unambiguously shows the centennial to millennial climatic variability, so-called Dansgaard-Oeschger (D-O), generally found in the Northern Hemisphere high-latitudes during the last glacial period. Strikingly, the BJSCK07 view of the Lake Malawi record suggests that abrupt climatic changes in tropical Africa lead similar abrupt climatic changes in Greenland by ca. 1200 years and are integrated in the thermal bipolar seesaw mechanism. However, here I propose that alternative palaeo-environmental interpretations may give a new perspective, less speculative, for this palaeoclimatic record. [2] The Zr:Ti peaks in the Lake Malawi record show an apparent good correlation with the D-O interstadials (warming) in the NGRIP dO record as formerly hypothesized by Brown et al. [2006]. Likewise, the main trends in the Lake Malawi Zr:Ti record seem to be very similar to those in the NGRIP dO record. However, BJSCK07 have correlated the Zr:Ti peaks in the Lake Malawi record with the D-O stadials and the Heinrich events (cooling). [3] One of the greatest problems encountered by BJSCK07 when trying to match the Zr:Ti peaks in the Lake Malawi record with the D-O interstadials results from their prior interpretations of the Nb:Ti ratio in the Lake Malawi sediments (which mimic the Zr:Ti ratio: the niobium and zirconium are incompatible elements, they are common in volcanic rocks where they are relatively enriched in the volcanic glass matrix in comparison to the more mobile titanium element). The Nb:Ti peaks have been previously interpreted by Brown and Johnson [2005] and Johnson et al. [2002] as the result of northerly winds blowing over the cool and dry volcanic landscape of the Rungwe area (northern part of the Lake Malawi catchment), which would have transported volcanic ash to the lake, resulting in higher Nb:Ti in lake sediments. According to BJSCK07, this scenario is not mechanistically consistent with climatic conditions corresponding to the D-O interstadials, since during these ‘warm’ events, the Inter Tropical Convergence Zone (ITCZ) was probably located to the north and the atmospheric circulation was consequently dominated by southeasterly winds in the Lake Malawi basin. [4] Johnson et al. [2002] have formerly identified a strong Nb:Ti peak in the Lake Malawi sediments during the Younger Dryas event (YD), which was subsequently claimed to be a dry climatic period during which the ITCZ was located south of Lake Malawi and the regional atmospheric circulation was dominated by intense northerly winds. [5] However, recent pollen studies from a sediment-core retrieved at Lake Masoko, located in the heart of the Rungwe area, have shown that the most developed tree cover in this volcanic province during the last 45,000 years occurred during the YD [Garcin et al., 2006a]. This suggests that aeolian erosion from the Rungwe area, supposed to be the main source of niobium and zirconium for Lake Malawi, was reduced or non-existent during the YD. [6] Further, several centimetric tephra layers have been observed during the YD in the Lake Masoko sediments [Garcin et al., 2006b]. These tephra layers, which are strongly enriched in niobium (Y. Garcin and D. Williamson, unpublished data, 2006), are also likely to have contributed to the Nb:Ti peak recorded in the Lake Malawi sediments during the YD. However, none of these discrete tephra layers have been observed in the Lake Malawi sediment cores [Johnson et al., 2002], due to the distal position of the coring site at Lake Malawi relatively to the main Rungwe volcanic centers (ca. 80-100 km away). In conclusion, the complex YD period, which has been defined by BJSCK07 as a reference period for the interpretation of the 55,000year-long record, is probably too polluted by the volcanic activity from the Rungwe to infer the ‘normal’ behavior of the Nb:Ti and Zr:Ti in the Lake Malawi sediments. [7] BJSCK07 explain the fluctuations of the Nb:Ti and Zr:Ti ratios based on changes in the wind alone, and they refer to Brown and Johnson [2005] and Johnson et al. [2002] for more detail concerning their interpretations. However, no chemical analyses from the catchment rocks, soils and sediment loads of rivers, have been presented in either of these references to support the interpretation of the Nb:Ti and Zr:Ti ratios in the sediment cores. [8] The northern Lake Malawi basin is heterogeneous and consists of Precambrian basement overlaid by sediments and volcanics ranging from Permo-Trias (Karoo) to Neogene [Ebinger et al., 1993]. These rocks have probably very contrasted signatures of niobium, zirconium and titaGEOPHYSICAL RESEARCH LETTERS, VOL. 35, L04701, doi:10.1029/2007GL032399, 2008