Human impacts in Antarctica: trace-element geochemistry of freshwater lakes in the Larsemann Hills, East Antarctica

Fresh water lakes are found in basement rock basins in the Larsemann Hills, East Antarctica, during the summer months. These lakes constitute a relatively simple natural laboratory to investigate the effects of recent and well-documented anthropogenic impact on a “pristine” environment. Larsemann Hills freshwaters have extremely low salinity (typically <1‰), and contain very low concentrations of trace elements of environmental significance such as Pb, U, and Zn. Typical Pb concentrations range from less than 5 ppt to 250 ppt. Although trace metal concentrations appear to be higher in lakes situated in the vicinity of stations, they are consistently lower (by several orders of magnitude, for some elements) than Standard International Drinking Water Guidelines. The chemistry of the lake waters is dominated by sea-spray input. Consequently, it is primarily a function of geographical factors, such as distance from the shore and exposure to winds. Shallow-level groundwater and surface water also contribute to the lake chemistry. No evidence was found for contamination from global air circulation. Although contamination resulting from activities at the research stations is generally near or below detection levels, very low levels of trace metal anthropogenic contamination were found in the vicinity of some research stations. Received: 13 November 1998 · Revised: 23 March 1999 Accepted: 12 April 1999     

拉斯曼丘陵新生代剥露作用的裂变径迹证据

在西南极和横贯南极山脉地区,新生代裂谷和剥露作用非常普遍。但是,文献中很少记录东南极地区的新生代剥露作用。文中根据东南极普里兹湾拉斯曼丘陵地质样品的磷灰石裂变径迹年龄和热历史的模拟,认为在东南极海岸边缘存在新生代的隆升和伸展作用,其年龄为始于(49.8±12)Ma。该年龄略晚于西南极裂谷系的启动年龄(约60~50Ma)。由于差异隆升作用,在拉斯曼丘陵地区发育了更新的正断层作用--拉斯曼丘陵拆离断层的新活动,其年龄为约5.4Ma。东南极周缘新生代裂谷和伸展作用的普遍存在,是冈瓦纳裂解以来大陆分离和印度洋形成的结果。     

拉斯曼丘陵新生代剥露作用的裂变径迹证据

在西南极和横贯南极山脉地区,新生代裂谷和剥露作用非常普遍。但是,文献中很少记录东南极地区的新生代剥露作用。文中根据东南极普里兹湾拉斯曼丘陵地质样品的磷灰石裂变径迹年龄和热历史的模拟,认为在东南极海岸边缘存在新生代的隆升和伸展作用,其年龄为始于(49.8±12)Ma。该年龄略晚于西南极裂谷系的启动年龄(约60~50Ma)。由于差异隆升作用,在拉斯曼丘陵地区发育了更新的正断层作用——拉斯曼丘陵拆离断层的新活动,其年龄为约5.4Ma。东南极周缘新生代裂谷和伸展作用的普遍存在,是冈瓦纳裂解以来大陆分离和印度洋形成的结果。     

关于东南极拉斯曼丘陵夕线片麻岩类原岩恢复问题的讨论

南极拉斯曼丘陵及其邻区的(含)夕线片麻岩类的原岩可以是杂砂岩、亚杂砂岩、石英砂岩、泥质岩或页岩等。粘土岩或页岩之类的富铝沉积岩与富夕线片麻岩并没有直接对应关系。没有一种泥质岩的化学组成可以与富夕线片麻岩相对应。研究区内夕线石片麻岩的形成主要与黑云斜长片麻岩有关,影响夕线石出现的决定性因素是特定温压条件下的变形变质改造过程,而不是原岩成分。夕线石片麻岩在很大程度上是黑云斜长片麻岩经长英质组分迁移之后的滞留-残留体。夕线石化过程中岩石组分发生了改造,相关变质作用已经明显偏离等化学过程,基本上属于开放体系。原岩中Al2O3的含量也不能控制夕线石的出现与否,含夕线石的岩石未必富铝,反之亦然,岩石富铝也可以不出现夕线石;但是夕线石化过程往往是Al2O3相对增加的过程,这些认识对于夕线石片麻岩成岩环境的确定和原岩建造的重建都具有重要意义。     

Preliminary magnetic susceptibility characteristics of the Bharati promontory (Grovness Peninsula), Larsemann Hills,Prydz Bay region,East Antarctica

The coastal tract of the Prydz Bay region in the East Antarctica exposes Archean to Late Proterozoic magmatic and medium- to high grade (amphibolite — granulite facies) metamorphic rocks. The para- and ortho gneisses from the Bharati promontory (Grovness Peninsula) forming a part of the Larsemann Hills in the southern segment of Prydz Bay were investigated for magnetic characterization. In this small peninsula the upper amphibolite facies gneisses occur as NE-trending bands. The para-gneisses show a range of mineral assemblages (± cordierite ± sillimanite ±garnet) while ortho-gneiss mineralogy includes quartz, feldspar, biotite, garnet. All the lithological units in Bharati promontory contain ubiquitous magnetite, however, with wide variation in the volume proportions. This has resulted in a wide range in magnetic susceptibility (10^?4 to 10^?2 SI). Magnetic foliations show a correspondence with the general trend of lithounits (050° NE) and define a resulting geometry of mainly D₁ and D₂ foliations. The magnetic lineations show a preferred orientation with moderate easterly plunge (mean vector 093/36). The findings have implications for the magnetic field survey because such fabrics would impart a strong horizontal component of induced magnetization.     

东南极拉斯曼丘陵地区变质杂岩的层序与构造变形过程

东南极拉斯曼丘陵地区位于兰伯特裂谷东缘普里兹湾东岸,该地区主要出露一套麻粒岩相变质岩,前期对原岩时代、变质过程等进行了详细研究,但是对于变质杂岩的层序和变形过程研究相对薄弱。文章通过大比例尺地质填图,发现拉斯曼丘陵地区变质杂岩总体成层有序,在此基础上建立拉斯曼岩群,并将其划分成6个岩组,原岩形成时代为中元古代。拉斯曼岩群经历了格林维尔期和泛非期变质作用的叠加,变质程度均达到高角闪岩相-麻粒岩相。拉斯曼丘陵地区主体构造线方向为北东东—南西西方向,总体上构成往北东东方向翘起的复式向斜构造,几个岩组的分布也显示由东向西逐渐变新。东部米洛半岛一带明显叠加了北北西—南南东向的构造变形。研究表明,拉斯曼岩群经历了6次重要的构造变形,包括新元古代格林维尔期(D1)、新元古代—早古生代泛非期变质变形作用(D2,D3,D4,D5)以及中新生代伸展作用(D6)。目前岩石中保存的主变形面理是格林维尔期和泛非期两次构造热事件的复合型面理,主要是泛非事件形成,格林维尔期变形面理呈残留状。综合拉斯曼岩群变质年龄及早古生代进步花岗岩体形成时代,认为D2~D5变形时代为550~500 Ma左右。因此,拉斯曼丘陵地区变质变形特征显示,中元古代拉斯曼岩群经历了格林维尔期和泛非期两次重要的造山作用,以及冈瓦纳大陆的裂解。     

南极拉斯曼丘陵长英质片麻岩中夕线石的出溶现象

南极拉斯曼丘陵长英质片麻岩中的粗粒夕线石可能有内部出溶现象,出溶形成的矿物有磁铁矿、钛铁矿-赤铁矿和石英出溶矿物条纹,并有少量的斜方辉石.电子探针成分分析表明,本区夕线石高温结晶时不仅有Fe3 ,可能还有Fe2 、Mg2 和Ti4 的替换,铁氧化物质量分数可高达2.9%;随着温度的不断降低,固溶体互溶度也不断减小,大部分微量组分从夕线石中析出形成出溶结构,而且低温变体中稳定的替换元素以Fd3 为主.铁组分的类质同像替换对物理化学计算和夕线石矿化环境均有所影响,对夕线石晶胞参数a的影响很小,而对b,尤其对c的影响较大,与前人的结论有所不同.     

东南极拉斯曼丘陵夕线石榴二长片麻岩的格林威尔期变质作用和构造意义

东南极拉斯曼丘陵出露的中-高级变质岩是普里兹湾基底岩系的主要组成部分,是研究普里兹湾高级区组成与演化的理想对象。本文对拉斯曼丘陵米洛半岛地区的夕线石榴二长片麻岩进行了岩石学、相平衡模拟和地质年代学研究。研究结果表明拉斯曼丘陵夕线石榴二长片麻岩矿物组合为石榴子石+堇青石+斜长石+钛铁矿+夕线石+石英±黑云母±钾长石,经历了格林威尔期(~980Ma)高温变质,其峰期阶段(M1)的矿物组合为石榴子石+斜长石+钾长石+钛铁矿+夕线石+石英,变质温压条件为:~820℃、~9.6kbar。夕线石榴二长片麻岩的岩浆锆石比变质锆石具有更低的LREE和更高的HREE含量,其原岩的沉积年龄不早于~1073Ma,物源主要为中元古代岩浆岩(S-型花岗岩)。拉斯曼丘陵在格林威尔期经历了陆-陆碰撞,进而快速折返、剥蚀作用的过程。     

长英质高级片麻岩中夕线石的形成与变形-变质-深熔作用的关系 —— 以南极拉斯曼丘陵区为例

南极拉斯曼丘陵高级长英质片麻岩的夕线片麻岩中可有两类结构和变质矿物组合均有所不同的两种域,一种含夕线石部分对应于片理组合,另一种对应无夕线石的非片理化组合.岩石的变形尤其是破裂性裂隙的率先出现对于富夕线石部分的形成是必要的.在非破裂性片麻理岩石域中,中-低压/高温条件下黑云斜长片麻岩进变质发展的结果往往是形成Grt+Qtz±Opx组合.这两种不同的变质域的组合与应变分解造成的强应变带和弱应变域相一致.而且,夕线石的形成不是简单的变质早期矿物固相反应的结果,而是反应链上的一部分.其出现是由开放体系中组分的差异迁移造成的,这种差异迁移实际上是碱土金属迁出(淋滤)的过程,与变形相伴的流体活动使得SiO2发生强烈淋滤,残留组分中SiO2活度大为降低,并使长英质组分和镁铁质组分分凝,主要组分大都可以单独富集(集中)、形成复杂的矿物演化和分布.这种演化还可从MgO等碱(土)金属组分的外迁程度差异来理解.随着碱(土)金属丢失程度的减小,依次出现夕线石、石榴子石、斜方辉石和堇青石,或者说,不同的变质或分异阶段形成不同的矿物(组合):变形-变质起始阶段,碱(土)金属组分迁移初期残留形成夕线石,之后为镁(铁)质组分迁移,初期残留不透明钛铁氧化物,晚期残留组分形成堇青石.石榴子石-长英质组合为体系基本封闭情况下的结晶.此外,夕线石的形成往往标志着深熔作用的开始,一旦深熔作用发展完善,夕线石呈准稳定状态或趋于消失.拉斯曼丘陵与夕线石有关的长英质岩石经历了复杂的变形、变质和流体活动变化.     

东南极拉斯曼丘陵柱晶紫苏堇青麻粒岩中堇青石的矿物学特征

东南极拉斯曼丘陵晚元古代（１０００Ｍａ）高级变质杂岩中区域性产出一套粗粒柱晶紫苏堇青麻粒岩,其原岩为含硼的富镁铝泥质岩。岩石中结晶粗大的堇青石在矿物化学成分上属于镁堇青石,其成分显示不均匀且具较明显成分环带的特征,从核部到边缘,其ＸＭｇ（Ｍｇ／Ｍｇ＋Ｆｅ２＋）比值由０．８５５变化为０．８１６。温压计算结果表明,所研究堇青石形成的Ｐ－Ｔ条件约为０．７６～０．７２ＧＰａ和８６０℃～８３０℃（核部至边缘）。Ｘ－射线粉晶结构分析证明这种堇青石属于低位结构状态的堇青石,晶体结构扭曲指数Δ＝０．２８～０．３０,在自然界比较少见。实验室合成的低位结构状态的堇青石只在持续较长时间的１３００℃～１４００℃的高温条件下稳定存在。因此,这意味着低位堇青石可能是由很高温变质作用及缓慢冷却的条件下结晶形成,这对于研究该区的地壳演化具有极其重要的意义。     

Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°-79°E, 68°-70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south-west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south-west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth. This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre-metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion-driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.     

南极拉斯曼丘陵高级区假蓝宝石及有关矿物组合的形成过程

通过对南极拉斯曼丘陵含假蓝宝石矿物及有关组合的研究,得知假蓝宝石的成分以富镁、铁、铝但贫硅为特征,其粒间及粒内成分的变化均较为明显,这种变化主要表现为以钙契尔马克代换(Mg, Fe) + Si = 2Al为主的类质同象置换。在形成假蓝宝石的高级变质-深熔作用过程中,岩石矿物组合的变化主要是多阶段变质造成的,如浅-暗色组分发生了分离,其中含镁铁组分多的矿物较早结晶,而镁铁组分的分离使得相对富Fe的矿物较早结晶,即晚期形成的镁铁质矿物更富Mg组分,并形成递进演化的不同阶段的组合。假蓝宝石的出现与组分的活动有关,而不仅仅是由岩石成分决定的。变质矿物组合的多次递进演化,反映了活动组分的不断变化,以及体系组分相当程度的开放性。含假蓝宝石的变质岩石之原岩未必是富镁泥质岩。本区假蓝宝石成分的多变性主要是环境成分而不是pT变化所引起的,假蓝宝石形成于840~880℃,并没有达到超高温（>1 000℃）的条件,其形成与高级变质-深熔作用过程中组分渗透和扩散的共同作用有关。     

南极拉斯曼丘陵长英质变质岩中磷灰石-斜方辉石组合的岩石学意义

南极拉斯曼丘陵高级长英质变质岩中出现的磷灰石-斜方辉石(Ap-Opx)组合表明岩石经历了麻粒岩相条件下的变质作用,长英质片麻岩发生了强烈的脱水深熔作用。随着熔体的迁出,长英质片麻岩中的残留或者镁铁质岩石中的分异产物,均可形成Ap-Opx组合。残留矿物如磷灰石、金属氧化物等具有丰富的稀土元素,强烈影响着岩石中的稀土元素配分型式。磷灰石-斜方辉石组合的存在说明体系中有适量但不是很充足的有效钙质组分。由磷灰石、磁铁矿-钛铁矿、石榴石,直至斜方辉石,一方面反映了活动组分-残留组分的不断变化,另一方面,也说明高级变质作用过程中体系的失水程度逐渐增加;变质作用中富含该组合的岩石是经变质过程形成的特有岩石,没有固有的原岩可对应。     

Geogenic sources and sinks of trace metals in the Larsemann Hills,East Antarctica: Natural processes and human impact

To evaluate the extent of human impact on a pristine Antarctic environment, natural baseline levels of trace metals have been established in the basement rocks of the Larsemann Hills, East Antarctica.From a mineralogical and geochemical point of view the Larsemann Hills basement is relatively homogeneous, and contains high levels of Pb, Th and U. These may become soluble during the relatively mild Antarctic summer and be transported to lake waters by surface and subsurface melt water. Melt waters may also be locally enriched in V, Cr, Co, Ni, Zn and Sn derived from weathering of metabasite pods. With a few notable exceptions, the trace metal concentrations measured in the Larsemann Hills lake waters can be entirely accounted for by natural processes such as sea spray and surface melt water input. Thus, the amount of trace metals released by weathering of basement lithologies and dispersed into the Larsemann Hills environment, and presumably in similar Antarctic environments, is, in general, not negligible, and may locally be substantial.The Larsemann Hills sediments are coarse-grained and contain minute amounts of clay-size particles, although human activities have contributed to the generation of fine-grained material at the most impacted sites. Irrespective of their origin, these small amounts of fine-grained clastic sediments have a relatively small surface area and charge, and are not as effective metal sinks as the abundant, thick cyanobacterial algal mats that cover the lake floors. Thus, the concentration of trace metals in the Larsemann Hills lake waters is regulated by biological activity and thawing–freezing cycles, rather than by the type and amount of clastic sediment supply.     

The structure and origin of Prydz Bay and MacRobertson Shelf, East Antarctica

A marine geophysical survey in early 1982, conducted by the Australian Bureau of Mineral Resources, indicated that much of Prydz Bay is underlain by a sedimentary basin. Severe seismic multiples preclude an accurate estimate of total sediment thickness, but interpretation of the seismic and magnetic data suggest that it is probably at least 5 km. The trends of the southeast basin margin and of mild faulting and folding in the southwest indicate an overall NNE trend, roughly orthogonal to the continental margin.In the south of Prydz Bay, two series of seismic sequences are evident, separated by a mildly erosional unconformity. The lower series ranges from poorly- to well-stratified, has minor folding and faulting, and probably derives from continental and perhaps shallow marine pre-breakup sediments. The upper series is generally well-stratified, and prograded near the shelf edge; it probably represents shallow marine post-breakup sediments. The seabed is distinctly unconformable with the underlying sediments, implying both that much of the upper series sediments and some of the lower series sediments have been bulldozed off by advances of the Amery Ice Shelf, and that present sedimentation rates are very low. Possible thin moraines or tillites in the northeast part of the Prydz Bay are also attributed to these glacial advances.The continental slope and rise sedimentary section ranges from at least 3 km thick off Prydz Bay, to thin off the MacRobertson Shelf to the west, reflecting the more prolific sediment source in Prydz Bay. The deep water section includes several seismic sequences, the most distinctive being interpreted as sheet volcanics and turbidite fans. The deepest visible unconformity is locally strongly faulted and may separate the pre-breakup and post-breakup sediments.Indo-Antarctic breakup has been tentatively dated as Early Neocomian (130 Ma) and the E-W orientation of the resultant Antarctic coast invites interpretation of the Lambert Glacier-Prydz Bay structure as a possible failed rift arm of a triple junction. Direct information on the age and nature of the sediments under Prydz Bay is lacking, but Permian continental sediments cropping out at Beaver Lake, to the south, may correspond to the lower pre-breakup series. This interpretation gains some support from analogies with fault-bounded intracratonic basins in India which contain Permian to Triassic continental strata and which may have been juxtaposed prior to breakup. The upper series probably consists of Upper Cretaceous and Cainozoic sands and shales, with moraines or tillites at the top of the section.     

东南极拉斯曼丘陵长英质片麻岩的深熔作用与铁钛氧化物的聚集机制

东南极拉斯曼丘陵高级变质长英质岩石中铁钛氧化物的局部聚集与高级变质作用过程中的深熔作用有关,并非原岩富集这些组分。深熔作用造成惰性组分如铁钛氧化物滞留原地或略有聚集及活动性组分的迁移,而流体挥发组分优先聚集于熔体之中。当体系中水含量较低、处于不饱和状态时,深熔作用过程中形成局部"熔体",其结晶所成的浅色体不具低共结组分,没有熔体结晶结构,不是真正的熔体,可能是(准)熔体。较粗粒的浅色体或伟晶岩也是与深熔作用有关的产物,其形成早于花岗岩脉或岩体,而与花岗质岩浆分异无关。伴随(准)熔体的出现,体系中组分的萃取、分异效果较为明显,即可造成组分分异,形成截然不同的异地、二相分异结构,分别形成固相残留物(组成可以不固定)和(准)熔体相。固相残留体中富铝、铁组分,形成矽线石和铁钛氧化物团块,其中少或无挥发分;与此对应,短距离迁移浅色体中往往贫铁钛组分,可见石榴子石、偶见铁钛氧化物矿物。这种挥发分不饱和状态下的深熔作用基本属于封闭体系,整体失水不显著,高级变质岩中的一些特征矿物如矽线石、石榴子石、堇青石、尖晶石的形成也与这种分异作用有关,但组分迁移范围有限,并可保存组分分异各阶段的产物。拉斯曼丘陵长英质岩系中大量铁钛氧化物和矽线石类矿物组合的形成,反映了临界状态下的局部或差异抬升,变形作用的非均匀性及相伴随的组分分异作用,很可能相当于早期格林维尔期构造的泛非期再活动。      

东南极拉斯曼丘陵麻粒岩相岩石中早期残留矿物组合的特征及其变质作用条件

东南极拉斯曼丘陵出露的麻粒岩相泥质片麻岩和镁铁质麻粒岩经历了复杂的变形变质历史。代表前进的增厚事件的早期残留变质构造Ｄ１形成于１０００Ｍａ（Ｇｒｅｎｖｉｌｉａｎ）期间。对应于Ｄ１的变质峰期Ｍ１的变质组合以泥质片麻岩的石榴石和尖晶石变斑晶中的堇青石和夕线石包裹体以及镁铁质麻粒岩的石榴石变斑晶中的斜方辉石和斜长石包裹体为特征。在该区斯托尼斯半岛的紫苏黑云石英岩的粗粒紫苏辉石中发现了包裹的假蓝宝石＋磁铁矿和假蓝宝石＋尖晶石＋夕线石＋磁铁矿＋石英组合,这种假蓝宝石又包裹更细粒的尖晶石包裹体。石榴石－斜方辉石温度计及石榴石－斜方辉石－斜长石－石英压力计的计算表明Ｍ１变质作用最大的Ｐ－Ｔ条件为０．９５ＧＰａ和８７０℃。这些早期残留的矿物组合指示了其变质作用具有逆时针近等压冷却（ＩＢＣ）的Ｐ－Ｔ演化特征,反映了它们的形成是一个埋深期间前进加热的进变质作用过程。本文得出该区Ｍ１（１０００Ｍａ）变质作用的逆时针（ＩＢＣ）Ｐ－Ｔ轨迹可能与已存在的大陆壳下面的岩浆底侵作用及地壳内大量岩浆物质的侵入和结晶作用有关。此类型逆时针Ｐ－Ｔ轨迹不同于该区在晚期５００Ｍａ（Ｐａｎ－Ａｆｒｉｃａｎ）期间顺时针演化的Ｐ－Ｔ轨迹     

A preliminary record of the historical seabird population in the Larsemann Hills, East Antarctica, from geochemical analyses of Mochou Lake sediments

An 85-cm long sediment sequence from Mochou Lake on Mirror Peninsula in the Larsemann Hills, East Antarctica, was investigated for elemental geochemistry, biogeochemistry and strontium (Sr) isotopic composition in the acid-insoluble and acid-soluble fractions. The biogeochemical data showed that the freshwater lacustrine sediment originated after 2750 BP. The element Zn, Cu, F, P and Se fluxes displayed almost the same vertical distribution patterns, and their ratios over loss on ignition had strong positive correlations with each other, indicating a common source, probably from seabird droppings. The Sr isotopic compositions provided further evidence of the influence of seabirds on the freshwater lacustrine sediments. The ⁸⁷Sr/⁸⁶Sr ratios determined in the acid-soluble fraction were remarkably lower than those of the acid-insoluble fraction; with a bedrock ⁸⁷Sr/⁸⁶Sr signature, they were close to those of the marine-derived Sr and were negatively correlated with total amino acids. Using the ratio vs. depth profiles of P, Zn, Cu, Se, F, As, Cd and Ni fluxes over Al, we reconstructed the historical seabird population in the catchment area of Mochou Lake. During the past c. 2800 years, the seabird population has experienced substantial changes in the absence of human activity, and four troughs were identified. After comparison with the palaeoclimatic records and glacial advance and retreat history, the pronounced seabird population declines seem to be related to climatic deterioration.