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The distribution of long chain alkenones in modern lacustrine sediments in the Lake Qinghai and lakes from the Qaidam Basin

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Xiangzhong Li at Yunnan University
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This paper analysed the existence of long-chain alkenones from the surface sediments in the Lake Qinghai and lakes in Qaidam Basin. The ratios of C 37 to C 38 alkenone is observed larger than 1 in both freshwater and saline lakes, while less than 1 in salt lakes. The calculated temperatures based on 37 K U ′ equation reported in literatures can fall within the observed temperature ranges for the Lake Qinghai, but are lower than air temperatures observed for lakes in the Qaidam Basin. The abundances of the long-chain alkenones in the saline lakes are found higher than those in freshwater and salt lakes, with the highest abundance of 86.09g/L in Lake Gahai with salinity of 103.2g/L. Such characteristics suggest that the algal precursors might prefer to live in water bodies with intermediate salinity but do not grow well either in freshwater or salt lakes. Thus the fluctuation of long-chain alkenone abundance might indicate variations of freshwater or saline environments.
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J
. Lake Sci.(湖泊科学), 2008, 20(3): 285-290
http://www.jlakes.org. E-mail: jlakes@niglas.ac.cn
©2008 by Journal of Lake Sciences
青海湖及柴达木盆地地区现代湖泊沉积物中长链烯酮的分布特征*
付明义1,2, 刘卫国1,3, 李祥忠1,2, 徐黎明4, 王 政1, 安芷生1
(1: 中国科学院地球环境研究所, 黄土与第四纪地质国家重点实验室, 西安 710075)
(2: 中国科学院研究生院, 北京 100049)
(3: 西安交通大学, 西安 710075)
(4: 中国科学院盐湖研究所, 西宁 810009)
摘 要: 研究了我国青海湖和柴达木盆地地区不同类型湖泊表层沉积物中的长链烯酮. 结果表明该地区湖泊中可能广泛存在
长链烯酮. 多数淡水湖、咸水湖现代沉积物中C37/C38>1, 而盐湖中C37/C38<1. 根据文献中报道的湖泊环境下长链烯酮不饱和度
37
K
U′与温度的方程, 估计的温度在青海湖地区处于实际温度范围内, 而在柴达木盆地低于实际温度. 结果亦显示在青海湖和
柴达木盆地地区, 长链烯酮的含量在咸水湖中要明显的高于淡水湖和盐湖, 最高含量出现在咸水湖(盐度为103.2g/L)尕海中,
高达86.09µg/g; 这种含量和分布特征可能表明生产长链烯酮的藻类应该更喜欢生长于具有一定盐度的环境; 长链烯酮含量的
变化可能指示咸水或淡水环境.
关键词: 长链烯酮; 青海湖; 柴达木盆地; 盐度; 丰度
The distribution of long-chain alkenones in modern lacustrine sediments in the Lake
Qinghai and lakes from the Qaidam Basin
FU Mingyi1,2, LIU Weiguo1,3, LI Xiangzhong1,2, XU Liming4, WANG Zheng1& AN Zhisheng1
(1: State Key Laboratory of Loess and Quaternary Geology, IEE, CAS, Xi’an 710075, P.R . C h i n a )
(2: Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R . C h i n a )
(3: Xi’an Jiaotong University, Xi’an 710049, P.R . C h i n a )
(4: Qinghai Institute of Salt Lakes, CAS, Xining 810009, P.R . C h i n a )
Abstract: This paper analysed the existence of long-chain alkenones from the surface sediments in the Lake Qinghai and lakes in
Qaidam Basin. The ratios of C37 to C38 alkenone is observed larger than 1 in both freshwater and saline lakes, while less than 1 in salt
lakes. The calculated temperatures based on 37
K
U′equation reported in literatures can fall within the observed temperature ranges for
the Lake Qinghai, but are lower than air temperatures observed for lakes in the Qaidam Basin. The abundances of the long-chain
alkenones in the saline lakes are found higher than those in freshwater and salt lakes, with the highest abundance of 86.09g/L in Lake
Gahai with salinity of 103.2g/L. Such characteristics suggest that the algal precursors might prefer to live in water bodies with
intermediate salinity but do not grow well either in freshwater or salt lakes. Thus the fluctuation of long-chain alkenone abundance
might indicate variations of freshwater or saline environments.
Keywords: Long-chain alkenone; Lake Qinghai; Qaidam Basin; salinity; abundance
长链烯酮系指一类含有 2-4个双键的直链不饱和甲基和乙基酮, 其C原子数在 37-39 之间, 广泛分
布于海洋沉积物中[1-2]; 主要是由定鞭金藻纲, 特别是该纲中的颗石藻 Emiliania huxleyi 及其它少数种属
如Gephyrocapsa oceanica 合成的一种特征性生物标记化合物[2-5]; 其C37 烯酮的不饱和度值(37
K
U′)已经被成
功地用来重建古海洋水表温度[6-9]. 近年来, 一些研究者把目光对准了湖泊环境中的长链烯酮, 期望通过
* 国家自然科学基金项目(40599422, 40523002, 40121303)和国家重点基础研究发展规划项目(2004CB720200)联合资助.
2007-06-08 收稿; 2007-07-27 收修改稿. 付明义, 男, 1982 年生; E-mail: 2005f868@sina.com.
J. Lake Sci.(湖泊科学), 2008, 20(3)
286
C37烯酮的不饱和度值重建湖泊区域气候的变化. 研究者们已在全球范围内的湖泊中发现了长链烯酮的
存在. Zink等[10]研究了俄罗斯、澳大利亚、德国及美国古代和现代湖泊中的长链烯酮, 并建立了德国湖泊
中37
K
U′与湖泊表层水温的线性相关关系. 在我国, Li等[11]发现了青海湖中存在长链烯酮; Wang等[12]和盛国
英等[13]研究了碱性碳酸盐型咸水湖和盐湖沉积物中的长链烯酮; 孙青等[14]研究硫酸盐型盐湖中的长链
烯酮, 发现咸水湖和盐湖沉积物中 37
K
U′与年平均气温存在良好线性关系; Chu等[15]研究了中国38个不同类
型湖泊中长链烯酮的分布、组成, 建立了长链烯酮不饱和度(37
K
U
′
)与年平均气温的线性关系. 在南美洲的
Titicaca湖, Theissen等[16]认为: 只要能够确定其母源, 长链烯酮不饱和度可以作为湖泊环境下温度的替代
指标. D’Andra等[17]和Liu等[18]依据已有的 37
K
U
′
与温度的关系方程分别重建了格陵兰湖和青海湖地区的温
度变化.
然而, 已有的研究对柴达木盆地地区现代湖泊中长链烯酮在不同类型水体中的组成、分布及丰度特
征尚缺乏较系统的认识. 本文从青海湖和柴达木盆地部分现代湖泊中采集了表层沉积物, 分析不同类型
湖泊中长链烯酮的分布、丰度及不饱和趋势, 为长链烯酮不饱和度在湖泊环境下的应用提供依据.
1 样品和实验
1.1 研究区概况和样品
青海湖(99°36´-100°47´E, 36º32´-37°15´N)是我国最大的内陆高原微咸水湖泊, 湖面海拔1981
年为3193.92m, 湖水矿化度1986年为13.84g/L[21]. 湖区年均气温1.2 , ℃多年平均降水量336.6mm,
属高寒半干旱气候. 湖区为大通山、日月山和青海湖南山所环绕, 有大小河流40条, 都属内陆封闭
水系. 此外, 在蒸发最强、补给最弱的东部湖滨, 出现三个子湖尕海(100°34´E, 37°00´N)、耳海和海
晏湾[21].
柴达木盆地(90º00´-98º20´E, 35º55´-39º10´N)居于青藏高原的东北侧, 是我国第三大内陆盆地. 四周
为阿尔金山、祁连山南麓和昆仑山北麓所环绕, 东西长约850km, 南北宽逾300 km, 面积达25×104km2. 海
拔分布在2675-3171m. 盆地内气候干旱, 多风少雨, 具有高原荒漠的气候特征. 年均气温为2-4.℃ 盆地
内现有湖泊33个. 其中淡水湖1个、半咸水-咸水湖7个, 盐湖15个. 此外, 还有一些无表面湖水的“干盐
湖”, 如大浪滩、一里平、察尔汗等. 盐湖的平均含盐量高达332.4g/L; 湖水一般都较浅, 面积多变[22].
样品采自青海湖及柴达木盆地的尕海、克鲁克湖、大苏干湖、小苏干湖、托素湖、大柴旦湖、小柴
旦湖及冷湖等湖泊表层0-2cm的现代沉积物(图1).
图1 青海湖及柴达木盆地采样湖泊分布(● 淡水湖、■ 咸水湖、▲ 盐湖)
Fig.1 Map of sampled lakes in Lake Qinghai and Qaidam Basin
付明义等: 青海湖及柴达木盆地地区现代湖泊沉积物中长链烯酮的分布特征
287
1.2 实验方法
样品采集后, 冷冻保存. 实验时, 样品在 40℃下烘干, 于玛瑙研钵中磨至 100 目, 称取 2g 左右, 在
ASE(Accelerated Solvent Extraction)200 上(温度 100℃, 压力 1500psi)加速抽提, 提取液为 9:1 的CH2Cl2和
MeOH 的混合液, 重复抽提 4次; 提取液在柔和的氮气下吹干, 加入 6％的氢氧化钾甲醇溶液 3ml, 超生震
荡10min, 室温放置过夜, 次日再超声震荡一次, 用正己烷萃取其中的非极性成分 4次; 在硅胶柱上分别利
用正己烷和二氯甲烷提取烷烃和酮类组分的溶液[24]; 含烯酮的馏分加入 C36 正构烷烃内标(即采用外标法)后,
在氮气下吹干, 溶于 20L 甲苯中, 超声震荡 1min 后进行 GC 分析.
气相色谱分析(GC)在Agilient 6890 气相色谱仪上进行, 柱头进样温度 310℃. 色谱柱为 HP-1MS 型毛
细管柱(60m×0.32mm×0.25m), 氦气作为载气, FID 检测. 升温程序为:初始温度 60℃下保留 1min, 然后以
15.00℃/min 升温至 200℃, 再以 5.00℃/min 升温至 270℃, 再以 2.00℃/min 升温至 300℃, 恒温 40.00min,
再以 5.00℃/min 升温至 310℃, 恒温 10.00min. 化合物的鉴定主要依据 GC 保留时间及与标准色谱图的对
比来确定[11,18]. 按长链烯酮各异构体与 C36 正构烷烃的响应因子为 1计算长链烯酮各异构体的浓度, 计算
公式为: 长链烯酮各异构体的浓度=C36 正构烷烃含量×长链烯酮各异构体的峰面积×1/C36 正构烷烃的峰
面积.
2 结果与讨论
2.1 长链烯酮的分布特征
青海湖和柴达木盆地的尕海、克鲁克湖、大苏干湖、小苏干湖、大柴旦湖、小柴旦湖、托素湖及冷
湖等湖泊的表层沉积物中均检测到了 C37-C39 长链烯酮.
不同盐度环境下, C37-C38 长链烯酮各异构体的组成和丰度呈现出相似的分布特征. C37 不饱和酮是由
C37:4、C37:3 和C37:2 3个异构体组成的; C38 不饱和酮一般是由 C38:4、C38:3 和C38:2 3个异构体组成. 具有相
同碳数的长链烯酮中, 含3个C=C 的长链烯酮(C37:3 或C38:3)在其各同系物中通常是丰度最高的. 研究湖
泊中, 多数的淡水和咸水湖(包括青海湖)沉积物中长链烯酮丰度随碳链的增长丰度逐渐变小(一般是
C37>C38>C39), 与开阔海洋[9,25]、临海[26-27]以及部分湖泊环境[10-15,28] 下长链烯酮的丰度变化顺序相同; 多
数盐湖沉积物中长链烯酮的丰度变化一般是 C38>C37>C39. C39 长链烯酮的丰度很低.
C37 和C38 不饱合酮的比值(C37/C38)在自然环境中, 特别是在开放海洋环境下一般都比较低; 而湖泊
环境下的 C37/C38 比值似乎变化范围较大[11,14,25]. C37/C38 比值可能受到环境状况、营养水平、藻类细胞的
生理状态和生长温度的影响[29]; Chu 等[15]认为: C37/C38 比值较大的变化还可能指示了生产烯酮的不同藻
类物种的输入. 在我们研究的青海湖和柴达木盆地地区, 多数淡水和咸水湖的沉积物中 C37/C38>1, 而大
柴旦湖、小柴旦湖等盐湖的沉积物中 C38 长链烯酮的含量较高, C37/C38<1, 普遍存在着两种模式(C37/C38>1
和C37/C38<1); C37/C38 比值变化很大(0.40<C37/C38<7.66), 可能暗示了青海湖与柴达木盆地湖泊中的长链
烯酮来源于不同的生产藻类.
2.2 长链烯酮不饱和度与温度的关系
青海湖及柴达木地区湖泊表层沉积物中长链烯酮的 37
K
U
′
值如表 1. 利用 Chu 等[15]的方程(37
K
U
′
=
0.0328T+0.126, R2=0.83), 得到的年平均气温在青海湖地区为 0.5℃, 而在柴达木盆地处于了冰点
以下, 为-0.7℃. 利用Zink 等[10]的方程(37
K
U
′
=0.02T-0.121, R2=0.89), 我们得到青海湖和柴达木盆地
地区湖泊的夏季年平均表层水温分别为 13.2℃和11.2 .℃ 与现代气象资料比较(近几十年来, 青海
湖地区年均气温在 1.2℃左右, 夏季年均气温在 13.2℃左右 [21,23]; 柴达木盆地年均气温大约为 2-5℃,
夏季年均气温在 15.0℃左右[22,30-31]), 青海湖地区的温度重现很好, 而柴达木盆地地区的恢复温度
偏低; 这可能是受到生产长链烯酮藻类的物种差异、繁殖期不同及其它环境因素(盐度、营养水平等)
的影响[16]; 同时, 也应该考虑到不同湖泊沉积速率的差异, 表层沉积物代表了不同的沉积时间, 可
能降低了 37
K
U′值与湖区气温的相关性及柴达木盆地特殊气候环境, 降水主要集中在夏季, 湖水面积
变化较大等因素. 这些都需要我们做进一步的工作来确定柴达木盆地地区长链烯酮不饱和度与温度
的关系.
J. Lake Sci.(湖泊科学), 2008, 20(3)
288
表1 湖泊特征及长链烯酮丰度
Tab.1 Characteristics of the lakes investigated and alkenone abundance
类型 湖泊名称 水化学类型[21] pH 水深(m) 盐度(g/L) C37(µg/g) C38(µg/g) C37+C38(µg/g) C37/C38 37
K
U′2)
淡水湖 克鲁克湖 硫酸钠亚型 8.0[21] 2.94[21] 1.0 0.17 0.15 0.32 1.10 0.05
咸水湖 托素湖 硫酸镁亚型 8.9 [21] 3-4[22] 15.9[21] 0.07 0.06 0.13 1.30 0.17
托素湖 氯化钠亚型 8.9 [21] 3-4[22] 15.9[21] 0.11 0.11 0.22 1.04 0.20
托素湖 硫酸钠亚型 8.9 [21] 3-4[22] 15.9[21] 0.09 0.15 0.24 0.62 0.12
青海湖 氯化钠亚型 9.3[15] 17.9[21] 13.9[15] 0.03[15] 0.02[15] 0.05[15] 1.33[15] 0.14[15]
青海湖 氯化钠亚型 9.23[21] 17.9[21] 16 13.99 2.0 16.02 6.87 0.15
青海湖 氯化钠亚型 9.23[21] 17.9[21] 16 13.44 2.0 15.46 7.66 0.14
青海湖 氯化钠亚型 9.23[21] 17.9[21] 16.2 4.67 0.6 5.29 6.71 0.14
小苏干湖 硫酸钠亚型 7.4[21] ―1) 3.5 0.15 0.12 0.27 1.22 0.18
小苏干湖 硫酸钠亚型 7.4[21] ―1) 3.5 0.09 0.08 0.17 1.12 0.11
大苏干湖 硫酸钠亚型 8.9[21] 2.00[22] 37.3 19.38 25.05 44.42 0.77 0.10
冷湖 硫酸钠亚型 8.2[21] ―1) 48.3 0.07 0.17 0.24 0.44 0.08
尕海 硫酸镁亚型 8.28[21] 8.00[22] 103.2 44.99 41.09 86.09 1.09 0.08
盐湖 大柴达木湖 硫酸镁亚型 157.95[21] 0.34[22] 280.6 0.13 0.16 0.29 0.84 0.06
大柴达木湖 硫酸镁亚型 7.95[21] 0.34[22] 280.6 0.15 0.18 0.33 0.81 0.08
小柴旦湖 钠、镁硫酸亚型 7.8[21] 0.26[22] 192.8 0.06 0.13 0.18 0.46 0.09
小柴旦湖 钠、镁硫酸亚型 7.8[21] 0.26[22] 192.8 0.05 0.08 0.12 0.66 0.08
小柴旦湖 钠、镁硫酸亚型 7.8[21] 0.26[22] 192.8 0.13 0.33 0.47 0.40 0.04
1)无数据; 2) 37
K
U′=[C37:2]/[C37:3+C37:2].
2.3 长链烯酮含量与盐度的关系
D’Andrea 等[17]发现在格陵兰湖泊中长链烯酮的分布似乎依赖于盐度, 长链烯酮在淡水湖中普遍没有
检出, 而出现在具有较高盐度的湖泊中. 然而, 通过对中国 50 个湖泊较系统的研究, Chu 等[15]发现: 长链
烯酮的含量在淡水湖、咸水湖和盐湖中并没有表现出明显差别; 但是, 长链烯酮在高盐度的硫酸盐型湖
泊中含量很低, 可能这类环境并不利于长链烯酮生产藻类的生长. 我们研究地区的湖泊包括了淡水湖、咸
水湖和盐湖三种类型, 盐度变化范围为 1.0-280.6g/L. 图2a 是沉积物中长链烯酮(C37+C38)含量与盐度的相
关关系(R2=0.0035), 可能指示了青海湖和柴达木地区湖泊中长链烯酮的含量似乎并不受控于盐度的变化.
经对比, 可看出: 咸水湖中长链烯酮的含量普遍比淡水湖和盐湖中的高1-2个数量级, 最高含量出现在咸
水湖(盐度为 103.2g/L)尕海中, 高达 86.09g/L; 低盐度环境下, 长链烯酮的含量与盐度(盐度≤103.2g/L)存
在一定的关系(R2=0.75)(图2b). 同时, 自从 Cranwell 等[19]在英国的淡水湖泊中发现了长链烯酮后, 报道
的湖泊环境中长链烯酮的分布也多在高蒸发量、无外流的封闭湖泊环境, 而这类环境通常是具有一定盐
度的. 结果显示: 长链烯酮生产藻类应该更喜欢生长于具有一定盐度的咸水或微咸水环境, 过高或过低
的盐度可能都不适合产生长链烯酮的藻类生长; 而合适的盐度环境下(如托素湖和冷湖)出现了低含量的
长链烯酮, 这可能是由于自然环境下别的因素限制了生产长链烯酮的藻类的生长.
3 结论
分析了我国青海湖和柴达木盆地地区不同类型湖泊表层沉积物中的长链烯酮. 结果表明我国青海湖
和柴达木地区湖泊中可能广泛存在长链烯酮. 多数淡水湖、咸水湖现代沉积物中C37/C38>1, 而盐湖中
C37/C38<1. 利用湖泊环境下长链烯酮不饱和度 37
K
U
′
与温度的相关方程, 估计的温度在青海湖地区处于实
际温度范围内, 而在柴达木盆地低于了实际温度. 青海湖和柴达木盆地地区, 咸水湖中长链烯酮的含量普
付明义等: 青海湖及柴达木盆地地区现代湖泊沉积物中长链烯酮的分布特征
289
图2 青海湖及柴达木盆地湖泊中(C37+C38 )长链烯酮丰度与盐度关系
(a: 所有研究湖泊; b: 低盐度湖泊)
Fig.2 Correlation between alkenone abundance (C37+C38) and salinity in Lake Qinghai and lakes from the Qaidam Basin
(a)all lakes investigated and (b)lower salinity lakes
遍比淡水湖和盐湖高, 而最高含量出现在咸水湖(盐度为 103.2g/L)尕海中, 高达 86.09g/g; 这种含量和分
布特征可能表明生产长链烯酮的藻类应该更喜欢生长于具有一定盐度的环境. 长链烯酮丰度的变化可能
指示咸水或淡水环境. 未来对更多青藏高原不同类型湖泊的研究将有助于深入的认识高原湖泊长链烯酮
地球化学意义.
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... b Alkenone concentrations in units of ng/L for surface waters and lg/g for surface sediments. c Alkenone concentration and U K 0 37 previously reported in Fu et al. (2008). d Data previously reported in Liu et al. (2008). 1 min, and subsequently programmed from 60 to 200°C at 15°C/min, to 270°C at 5°C/min, to 300°C at 2°C/min, and then to 310°C at 5°C/min, and finally held at 310°C for 10 min. ...
... Chu et al. (2005) sug-gested that alkenone concentrations from carbonate lakes are higher than those in sulfate lakes in China, while the highest alkenone concentrations were found in sulfate-type lakes in Spain (Pearson et al., 2008) and the United States (Toney et al., 2010). Fu et al. (2008) suggested that higher alkenone concentrations occur in brackish/saline lakes than in fresh and hypersaline lakes, based on surface sediment studies in the Qaidam Basin and Lake Qinghai regions. The lakes investigated in this study were mainly sulfate lakes (Zhang, 1987). ...
... The lakes investigated in this study were mainly sulfate lakes (Zhang, 1987). Consistent with the results of Fu et al. (2008), we found that lower alkenone concentrations tended to occur in fresh (Lake Xiaosugan) and hypersaline (Lake Dachaidan and Xiaochaidan) lakes. Therefore, our results suggest that lake waters which are too fresh or too hypersaline may not provide favorable conditions for alkenone-producing organisms. ...
Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China
Article
  • Apr 2011
  • GEOCHIM COSMOCHIM AC
Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to ∼100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C37/C38 ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C37:4 (the percentage of the C37:4 alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C37:4 index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C37:4 values need further investigation.
View
... High content of alkenones was reported in lakes with low temperature, high lake level, and high concentration of sodium and sulfate Zink et al., 2001). A particular range of salinity is also suggested for the prosperity of alkenoneproducing algae, indicating that fresh water or hypersaline environment might be unsuitable for algae living (Fu et al., 2008;Liu et al., 2011). More importantly, the alkenone content would be extremely low when the lake was extremely shallow or desiccated with seasonal or occasional water cover at the sampling location, which is used to interpret the alkenone content record in this study. ...
Solar imprints on Asian inland moisture fluctuations over the last millennium
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Solar irradiance changes are thought to play an important role in natural climate variability. How the hydrological conditions were affected by solar irradiance in westerly-controlled arid central Asia (ACA) on decadal/centennial timescales remains poorly understood because of the lack of high-quality records. Here, we integrate 1.2-year-resolution x-ray fluorescence (XRF) scanner-derived carbonate accumulation estimates with 6-year-resolution biomarker and magnetic records in a well-preserved shoreline core from Lake Manas, northwestern China, to reconstruct lake level fluctuations and potential solar imprints over the last millennium. Besides the generally confirmed cool-wet/warm-dry climate pattern in ACA, our data also consistently show frequent and substantial lake level fluctuations, resembling solar activity changes, especially during the ‘Little Ice Age’. Wavelet spectral analyses of our XRF data indicate strong 8- to 16-year, 64- to 128-year and 128- to 256-year cycles, coinciding with the ~11-year Schwabe cycle, ~70- to 100-year Gleissberg cycle, and the ~200-year Suess-de Vries cycle. We therefore suggest the existence of solar imprints on effective moisture fluctuations in ACA over the last millennium, and the potential occurrence of the Schwabe cycle even during the solar minima.
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... 2000 mm (Zhao et al., 2010). Currently in the basin there is 1 freshwater lake, and 7 brackish to saline and 15 hypersaline lakes (Fu et al., 2008). Samples of surface (0–1 cm) sediment were collected from 12 lakes with a gravity corer or as grab samples from the shallow lakes. ...
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The chain length of long chain alkenones (LCAs) in ocean and lake sediments commonly ranges from C37 to C39. Alkenones > C40 are extremely rare: C41 and C42 alkenones have only been reported in a Cretaceous black shale (Cenomanian, ca. 95 Ma) from the Blake Bahama Basin, western North Atlantic (DSDP Leg 76, site 534). The significance of these exceptionally long chain alkenones is, however, unclear. W have conducted a systematic survey of LCAs in surface sediments from 12 lakes with a wide range of salinity (0.7 to 146.3 g/l) from arid northwestern China. They showed a general trend of increasing maximum chain length with increasing salinity. In particular, C41 and C42 LCAs occurred only in the surface sediments of hypersaline lakes (113.6 to 146.3 g/l), but were absent from lakes with lower salinity, indicating that these compounds may be produced by an unknown haptophyte species living in hypersaline environments.
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... For example, Li et al. [9] were the first to report the presence of LCAs in Lake Qinghai, the biggest inland lake of China; they suggested that the alkenones in the lake could be used for paleoclimatic studies. Later studies confirmed that LCAs are widely distributed throughout the salt lakes of the Tibetan Plateau [14,28]. A study of the general relationship between U 3 K 7 ′ in lake sediments in China and the mean annual air temperature was performed, in which the salt lakes in the Tibetan Plateau were included [15]. ...
Calibration of the U 37 K′ index of long-chain alkenones with the in-situ water temperature in Lake Qinghai in the Tibetan Plateau
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Full-text available
  • Mar 2013
  • CHINESE SCI BULL
Long-chain alkenones (LCAs) can potentially be used as indicators to understand past variations in lacustrine environments. Previous research has suggested that the relationship between the temperature and the unsaturation index of LCAs should be calibrated individually, because of the possible variations in the alkenone-producing algal species in the lacustrine environment. In this work, we have calibrated U 37K′ of water filter samples against the in-situ water temperature in Lake Qinghai, Tibetan Plateau. There are significant relationships between U 37K′ and the water temperature, a non-linear relationship was derived. Because the U 37K′ values did not respond sensitively at lower temperatures, we suggested that a quadratic regression (U 37K′=0.0011×T 2-0.0201×T+0.1959, n=15, r 2=0.74) was appropriate than linear regression to represent the relationship between the in-situ temperatures and U 37K′. Meanwhile, the U 37K correlation relationship was not more significant than U 37K′ index in our study. Because of the C37:4 effects by salinity change, we suggest U 37K is not as robust as the U 37K′ index as a temperature proxy, at least for the salt lake in the Tibetan Plateau. The calibration of the U 37K′ index in this work has provided a new understanding of historic climatic changes in the Tibetan Plateau.
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Molecular phylogeny and morphological diversity of inland Cladophora (Cladophorales, Ulvophyceae) from China
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Cladophora has a global distribution and provides a habitat and food for numerous organisms. However, the taxonomy of Cladophora is confusing and poorly defined. In this study, 69 freshwater and brackish-water Cladophora samples from inland China were collected, and both morphological and phylogenetic analyses of these samples were performed. The concatenated molecular phylogenies based on ribosomal DNA sequences showed that these inland samples belonged to two groups: freshwater Cladophora and non-freshwater Cladophora. The phylogenetic results showed that there were eight clades in the freshwater group. The results of morphological observation showed characteristic similarities among the clades, and the samples in each clade exhibited different degrees of phenotypic plasticity. The prediction model for internal transcribed spacer 2 secondary structure showed that freshwater Cladophora contain one ring structure with three main helices (helices I–III), in which the helix III-2 region was most variable. The high phenotypic plasticity and lack of diagnostic characteristics of the freshwater Cladophora group made delineating species difficult and challenging. Further sampling from a broader range in combination with more efficient molecular markers are necessary to clarify the taxonomy of Cladophora in the future.
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Environmental controls on long-chain alkenone occurrence and compositional patterns in lacustrine sediments, northwestern China
Article
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Long chain alkenones occur widely in lacustrine settings and their compositions are potential proxies for reconstructing terrestrial paleoclimatological and paleoenvironmental changes. However, factors influencing their occurrence, abundance, compositional pattern and unsaturation indices are not well understood. Here we investigated the alkenones in surface sediments from 19 lakes in northwestern China as well as potential environmental factors. Of the lakes, 12 contain long chain alkenones, 3 have the isomeric C37:3 alkenone and 3 have very low abundance of alkenones. Our results show that salinity has a substantial influence on the occurrence, concentration and compositional pattern. Most of the compositions are dominated by tri-unsaturated compounds, with C38 methyl ketones absent. The predominance of C37:4 and the occurrence of a C37:3 isomer are suggested to be responses to low salinity as both are only found in the three alkenone-containing freshwater lakes. The proportion of C37:4 varies by up to 50% and shows a negative relationship with salinity within the surveyed region. An arched relationship of alkenone content and salinity is apparent, with no or very low contents of alkenones in freshwater (salinity <3 g/L) and hypersaline (>100 g/L) environments. The U37K' index is suggested to be a better terrestrial temperature proxy than U37K as U37K' values from lakes containing elevated alkenone content are all within the ranges of published U37K'-temperature calibrations, and U37K values show a weak positive relationship with salinity.
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Occurrence and environmental significance of long-chain alkenones in Tibetan Zabuye Salt Lake, SW China
Article
Full-text available
  • Dec 1997
Long-chain alkenones, a group of sensitive organic molecular indicators of water temperature, have been rather extensively studied in marine environments. However they have never been systematically examined in lacustrine environments, despite reports of their occurrence in lake sediments. Here, we report on a recent study of long-chain alkenones in a sedimentary core from a high altitude (∼ 5400 m) salt lake, Zabuye Salt Lake (ZSL), Tibet. This is a critical location for global climate studies, especially of the atmospheric circulation of the North Hemisphere. C_37–C_39 methyl and/or ethyl alkenones, usually dominated by components with tetra- and tri-double bounds, are commonly the major components of the polar fraction of the extracted organic matter from most sections. Down-core (vertical) variation of alkenone indices, the measure of molecular unsaturation, is compared primarily with other environmental signals, including lithology, elemental and mineral compositions, and stable carbon isotopes of hydrocarbon biomarkers of this core. Down-core profiles of alkenone climatic indices (U^k_37and U^k_38) suggest ZSL had two warm periods, one during ∼ 20 – 30 ka (∼Jabula Interglacial Optimum) and another at 8 – 5 ka (Middle Holocene Optimum), and a severe cold period from ∼ 18 – 11 ka (Last Glacial Maximum). The proposed warmer intervals are generally characterized by higher contents of carbonate, organic carbon, alkenones and heavier ¹³C values of n-alkanes.
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Lipid biomarkers of the Haptophyta
Article
Full-text available
  • Jan 1994
Conte, M.H., Volkman, J.K. and Eglinton, G. (1994). Lipid biomarkers of the Haptophyta. In The Haptophyte Algae (eds J.C. Green and B.S.C. Leadbeater), pp. 351-377. Clarendon Press, Oxford.
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Clean-up procedures for the unbiased estimation of C37 alkenone sea surface temperatures and terrigenous n-alkane inputs in paleoceanography
Article
  • Jan 1997
  • J CHROMATOGR A
Sample preparation procedure for the quick determination of C25C33n-alkanes and the U37k index in small sediment sample amounts (ca. 1 g dry weight) have been developed. Alkaline hydrolysis and silica column fractionation are the two clean-up steps considered. The former eliminates the interferences from the sedimentary wax esters which dominate the lipid composition in high latitude Atlantic Ocean sediments. The latter has been designed to collect both lipid groups (C25C33n-alkanes and C37 alkenones) in one single fraction avoiding the interferences from highly polar compounds which affect, in particular, the quantitative determination of the n-alkanes. The whole procedure has been shown not to affect the U37k measurements, even at low C37 ketone concentrations such as 10 ng which is the minimum threshold of total C37 alkenones in the gas chromatographic system for avoidance of adsorption-related interferences [1]. As shown with real case examples the usefulness of these clean up methods is highly dependent on the amounts of interfering lipid compounds present in the sediment extracts and their application may not be necessary for the open sea sediments present in most low latitude areas.
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Calibration of alkenone unsaturation ratios (U[sup k[prime]][sub 37]) for paleotemperature estimation in cold polar waters
Article
  • Jan 1993
  • GEOCHIM COSMOCHIM AC
C[sub 37]-C[sub 39] long-chain unsaturated ketones (alkenones) and alkenes were identified in samples of particulate organic matter obtained from surface waters spanning the temperature range [minus]0.7-12.2[degrees]C in the Southern Ocean south of Australia. The carbon number distribution indicates that these compounds are derived from the prymnesiophyte alga Emiliania huxleyi which is the predominant coccolithophorid in these waters. Values of U[sup k[prime]][sub 37] were calculated according to the definition U[sup k[prime]][sub 37] = [37:2]/([37:2] + [37:3]), where [37:[chi]] is the concentration of the C[sub 37] alkenone with [chi] double bonds, and plotted against sea surface temperature (SST). The data form a contiguous set with earlier compilations of data from warmer waters. Over much of the temperature range the relationship between U[sup k[prime]][sub 37] and temperature is remarkably linear, and is best fitted by the simple equation U[sup k[prime]][sub 37] = 0.0414T - 0.156 (for temperatures 4-25[degrees]C). This calibration of U[sup k[prime]][sub 37] with temperature for field samples is significantly different from that established from analyses of prymnesiophyte algae in culture, and it diverges especially at the colder temperatures. Data for temperatures below about 4-6[degrees]C show increased scatter and the calibration at these low temperatures may be better fitted by a nonlinear exponential function, but this requires further testing. Certainly, the predictive power of the linear calibration is severely constained at temperatures less than 5.0[degrees]C, but at temperatures above this the data establish the usefulness of U[sub k[prime]][sub 37] as an SST indicator for waters north of the polar front in the Southern Ocean and for several regions of the world ocean.
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Atlantic core-top calibration of the U^K
Article
  • Jan 1995
A field calibration of the U K 37 index with sea surface temperature is discussed, through analysis of an extensive suite of surface sediments ( n = 109) from the northeastern Atlantic (2°S-75°N). Values of U K 37 are compared with sea surface temperatures for overlying waters measured at different depths and seasons, to obtain a correlation suitable for palaeotemperature reconstructions. The best fit is obtained using surface (0 m) temperatures corresponding to caloric winter and autumn months. However, the annual average, spring and summer surface temperature equations also have high correlation coefficients, and are also appropriate for climatic studies. The results further validate the general applicability of the U K 37 as a climatic proxy, because the calibration equations are valid over a wide range of surface water temperatures (0-28°C) for different algal populations and are representative of the average contribution of alkenones to sediments, as found in sediment cores.
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Long-chain alkenones in sulfate lakes and its paleoclimatic implications
Article
  • Oct 2004
The proxy of U 37 K′ index is widely applied to reconstructing paleotemperature in the marine environment. However, the long-chain alkenones (LCAs) research is rarely reported in the liminic system. In this paper, we studied LCAs in the surface sediments (0–10 cm) of nine sulfate lakes in the Inner Mongolia Autonomous Region, Xinjiang Uygur Autonomous Region and Qinghai Province, China. LCAs are detected in eight of the nine lakes, except a dry salt lake — Lake Lop. The result shows that LCAs might be universal in the sulfate lakes. In most of the surface samples, the sequence of the abundance of the long-chain alkenones is C37>C38C39>C40· And C37:4 methyl ketone is a predominant compound in the C37 alkenone homologue. Based on regression analysis of U 37 K′ -ratios and the average annual, seasonal (spring, summer, autumn, winter) temperatures, the linear relativity of U 37 K′ -ratios and mean annual temperatures is superior to the others, and the correlation coefficient is 0.88 (r 2= 0.88). The linear relationship of U 37 K′ and the average annual temperature in different water chemistry environments suggests that the mechanisms controlling U 37 K′ index in the saline lakes might be the same as those in the ocean environment. The alkenone unsaturation could become an important paleotemperature proxy in the lacustrine environment.
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Calibration of alkenone unsaturation ratios (Uk'37) for paleotemperature estimation in cold polar waters
Article
  • Apr 1993
  • GEOCHIM COSMOCHIM AC
C37–C39 long-chain unsaturated ketones (alkenones) and alkenes were identified in samples of particulate organic matter obtained from surface waters spanning the temperature range -0.7–12.2°C in the Southern Ocean south of Australia. The carbon number distribution indicates that these compounds are derived from the prymnesiophyte alga Emiliania huxleyi which is the predominant coccolithophorid in these waters. Values of Uk'37 were calculated according to the definition , where [37:x] is the concentration of the C37 alkenone with “x” double bonds, and plotted against sea surface temperature (SST). Our data form a contiguous set with earlier compilations of data from warmer waters. Over much of the temperature range the relationship between Uk'37 and temperature is remarkably linear, and is best fitted by the simple equation Uk'37 = 0.0414T — 0.156 (for temperatures 4–25°C). This calibration of Uk'37 with temperature for field samples is significantly different from that established from analyses of prymnesiophyte algae in culture, and it diverges especially at the colder temperatures. Data for temperatures below about 4–6°C show increased scatter and the calibration at these low temperatures may be better fitted by a nonlinear exponential function, but this requires further testing. Certainly, the predictive power of the linear calibration is severely constrained at temperatures less than 5.0°C, but at temperatures above this our data establish the usefulness of Uk'37 as an SST indicator for waters north of the polar front in the Southern Ocean and for several regions of the world ocean.
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Long chain alkenones in hypersaline and marine coastal microbial mats
Article
  • Jun 2005
  • ORG GEOCHEM
Long-chain alkenones in microbial mats from Mediterranean solar salterns and coastal areas from the Mediterranean Sea and North Sea have been studied. These ecosystems exhibit a distinct C37–C40 alkenone composition characterized by a predominance of triunsaturated compounds in all C37–C40 homologues and by a specific correspondence between carbonyl position and chain length. Thus, C37 and C39 homologues are methyl ketones and C38 and C40 homologues are ethyl ketones. The distribution also contains tetraunsaturated alkenones in significant proportion, particularly among the C37 homologues. The double bonds are located at Δ8,15,22,29 and Δ9,16,23,30 in the methyl and ethyl ketones, respectively, indicating that the number of carbon atoms between the carbonyl group and the first double bond remains constant.This distribution is different from C37–C40 alkenone mixtures previously found in open sea waters or freshwater environments. It suggests a biological origin distinct from that previously described for marine algal cultures or recent or ancient open sea sediments. The uniformity of the distributions, despite the strong salinity differences considered in the present study (salinity of 35–200) is remarkable. Depth analysis of the microbial mats from hypersaline environments shows fluctuations in total alkenone concentration, but no defined trends involving selective changes in compounds with a specific degree of unsaturation or chain length.
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Temperature dependency of long-chain alkenone distributions in Recent to fossil limnic sediments and in lake waters
Article
  • Jan 2001
  • GEOCHIM COSMOCHIM AC
Distribution patterns of C37 and C38 polyunsaturated long-chain alkenones (LCAs) serve as proxies for the determination of paleotemperatures for marine surface waters. We studied Recent/Subrecent and Late Glacial/Holocene sediments from Germany, Austria, Russia, and the U.S. to test for a correspondence between LCA distribution and surface water temperature in limnic systems. Previously, reports of LCA occurrence were restricted to sediments of 6 wide distributed freshwater and alkaline lakes. In this study 13 of 27 investigated lakes contained LCAs in surface sediments with concentrations varying between 12 to 205 μg/g TOC. Late Glacial to Holocene sediment sequences from Lake Steisslingen and Lake Wummsee, (Germany), Lake Pichozero (Russia), and Brush Lake (U.S.A.) contained abundant LCAs with averaged concentrations of 33 to 7536 μg/g TOC. For the first time we observed the occurrence of LCAs within in the water column of oligotrophic Lake Stechlin (NE-Germany). Alkenones were restricted to the zone of maximum chlorophyll concentration within the water column indicating that LCAs have a biosynthetic origin and can be attributed to phototrophic (micro)algae. Attempts to identify the producing organism, however, were not successful. Culture experiments allow various phytoplankton to be excluded as producers. Alkenone-producing algae are evidently of small size, hindering microscopical identification. LCAs commonly occur in high concentrations in Late Glacial sediments, mainly during the cold period of the Younger Dryas, whereas the Holocene usually is devoid of polyunsaturated alkenones. The episodic occurrence of LCAs restricts their utility as proxies for continuous geological records. Furthermore, lack of microscopical verification and the episodic distribution allow for different producers of unsaturated alkenones in Recent and Late Glacial sediments. An empirical relationship between LCA distribution and temperature was observed. In fossil sediments from Lake Steisslingen, there is a good correspondence between Uk37 and the temperature-controlled δ18O isotope ratio of lake chalk. Comparison of LCA patterns obtained from the uppermost centimetres of lake sediments with averaged summer surface water temperatures of the lakes studied, demonstrates a trend of covariance (r2: Uk′37 = 0.90, Uk37 = 0.67; n = 9). Hence, the same mechanism that causes temperature-dependence of LCA patterns in marine systems might be effective in limnic settings. Identification of alkenone producers and their culture under controlled temperature are still mandatory before LCAs can be routinely applied as paleotemperature proxy in limnic systems.
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Unusual distributions of long-chain alkenones and tetrahymanol from the highly alkaline Lake Van, Turkey
Article
  • May 1997
  • GEOCHIM COSMOCHIM AC
Long-chain C37 to C40 alkenones with di-, tri-, and tetra-unsaturation are very abundant in sediment trap material and Holocene to Late Pleistocene core samples from the Earth's largest soda lake, Lake Van (Turkey). Thus, the known distribution range of these typical biomarkers for haptophyte microalgae is extended to highly alkaline environments. The observed unsaturation patterns differ strikingly from those found in open marine haptophytes and sediments by an enhanced relative abundance of the tetra-unsaturated compounds, especially the C37:4 methyl ketone. Their preponderance is suggested to be a facies marker pattern for lacustrine and marginal marine areas of sedimentation. Using published U37K calibrations, no reliable absolute temperatures were obtained for the Lake Van samples. Accordingly, marine sea surface temperature determinations based on long-chain alkenones should be applied with caution when a contribution of these compounds from coastal or nonmarine sources can not be excluded. The presence of tetrahymanol and gammacer-3-one in the Lake Van materials is attributed to organic matter contributions of ciliates. The relative abundance of long-chain alkenones and of tetrahymanol/gammacer-3-one is considered to reflect changes in the environmental conditions, in particular in the hydrological setting. We suggest that times of pronounced stagnation are recognised by very high tetrahymanol/gammacer-3-one concentrations together with drastically increased stanol/stenol ratios, and intervals of enhanced convection or of high freshwater input are characterised by high alkenone contributions.
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