Terrigenous sedimentation processes along the continental margin off NW Africa: implications from grain-size analysis of seabed sediments

The terrigenous fraction of seabed sediments recovered along the north‐west African continental margin illustrates spatial variability in grain size attributed to different transport mechanisms. Three subpopulations are determined from the grain‐size analyses (n = 78) of the carbonate‐free silt fraction applying an end‐member modelling algorithm (G. J. Weltje, 1997). The two coarsest end‐members are interpreted as representing aeolian dust, and the fine‐grained end‐member is related to fluvial supply. The end‐member model thus allows aeolian fallout to be distinguished from fluvial‐sourced mud in this area. The relative contributions of the end‐members show distinct regional variations that can be related to different transport processes and pathways. Understanding present‐day sediment dispersal and mixing is important for a better understanding of older sedimentary records and palaeoclimate reconstructions in the region.     

Deciphering signals of late Holocene fluvial and aeolian supply from a shelf sediment depocentre off Senegal (north‐west Africa)

High‐resolution climatic records of the late Holocene along the north‐west African continental margin are scarce. Here we combine sediment grain size, elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallow‐marine sediment depocentre in the vicinity of the Senegal River mouth. The aim is to understand how these terrigenous components reflect climate variability during the late Holocene. Major element contents were measured and mineral identification was performed on three sub‐fractions of our sediment core: (i) fluvial material < 2 µm, (ii) aeolian material of 18–63 µm and (iii) a sub‐fraction of dual‐origin material of 2–18 µm. Results show that more than 80% of the total Al and Fe terrigenous bulk content is present in the fluviogenic fraction. In contrast, Ti, K and Si cannot be considered as proxies for one specific source off Senegal. The Al/Ca ratio, recording the continental river runoff, reveals two dry periods from 3010 to 2750 cal a BP and from 1900 to 1000 cal a BP, and two main humid periods from 2750 to 1900 cal a BP and from 1000 to 700 cal a BP. The match between (i) intervals of low river runoff inferred by low Al/Ca values, (ii) reduced river discharge inferred by integrated palynological data from offshore Senegal and (iii) periods of enhanced dune reactivation in Mali confirms this interpretation. Copyright © 2011 John Wiley & Sons, Ltd.     

Holocene climate variability in Sicily from a discontinuous stalagmite record and the Mesolithic to Neolithic transition

Fabric and stable isotopic composition of a Holocene stalagmite (CR1) from a cave in northern Sicily record changes in paleorainfall in the early Holocene. High δ¹³C stable isotope values in the calcite deposited from ca. 8500 to ca. 7500 yr ago are interpreted as reflecting periods of high rainfall. The wet phase was interrupted by two periods of multi-century duration characterized by relatively cool and dry winters centered at ca. 8200 and ca. 7500 yr ago, highlighted by low δ¹³C and δ¹⁸O values. A high variability of δ¹³C values is recorded from ca. 7500 to ca. 6500 yr ago and indicates that the transition from a pluvial early Holocene to the present-day climate conditions was punctuated by decadal-scale periods of relatively dry winters. In northern Sicily, the traditional elements of the Neolithic appear at ca. 7700 yr ago. It is possible that changes in rainfall influenced the passage from hunter-gathering to farming and sheep-herding economies.     

A mid to late Holocene record of palaeoclimatic changes from Nal Sarovar: a palaeodesert margin lake in western India

With a view to obtain palaeoclimatic data from a climatically sensitive region we have studied core samples from Nal Sarovar, a large shallow lake lying within the palaeodesert margin of Thar in western India. A combination of C/N ratios and δ¹³C on a radiocarbon-dated core section have been used as climatic proxies. A high-resolution record extending back to ca. 6.6 ka BP has been reconstructed. The data indicate that, in the past, climate has varied from drier to wetter than present on time-scales of few hundreds to few thousands of years. There are, however, significant differences on the timing of these wet and dry periods, when compared with the available data from lakes farther north, in Rajasthan. Further, it looks unlikely that during the 6.6 ka the catchment areas of Nal Sarovar experienced such a significant increase in rainfall as has been suggested for the Rajasthan lakes. Interestingly, drier periods in Nal Sarovar data appear to correlate well with periods of glacier expansion in Eurasia indicating that the palaeoclimatic variations recorded in Nal Sarovar are a regional feature. © 1997 John Wiley & Sons, Ltd.     

The grain-size characterisation of coastal sand from the Somme estuary to Belgium: Sediment sorting processes and mixing in a tide- and storm-dominated setting

Sand-rich Holocene to modern clastic deposits in the eastern English Channel and the southern North Sea coasts of France and Belgium occur extensively as nearshore-sand bank, estuarine-tidal flat, aeolian dune and beach sub-environments. Sand samples (n = 665) collected from these deposits suggest the presence of three different populations: a largely dominant (83%) medium to fine quartz sand population (“b”), and finer- (14%) and coarser-grained (4%) populations (respectively “c” and “a”). The distribution of these populations among the four sub-environments reflects tide- and storm-dominated sorting and transport processes and a variable degree of mixing. These populations are derived from a mixture of very fine- to very coarse-grained fluvial, outwash and paraglacial sediments deposited on the beds of the eastern English Channel and southern North Sea during the late Pleistocene lowstand. The nearshore-sand bank environment, which also corresponds to the main offshore source area of the coastal deposits, exhibits population heterogeneity reflecting the variability of hydrodynamic conditions and sediment sorting in this zone. The nearshore topography of tidal ridges, banks and troughs in these tidal seas leads to variable bed and tide- and storm-induced shear stress conditions. These conditions only allow for the mobilisation and onshore transport of some of the finer fractions (populations “b” and “c”), leaving an offshore mixture of these finer populations with coarser, less mobilisable sediments (population “a”). Once in the coastal zone, these two finer populations undergo further hydrodynamic sorting and segregation. Variably sorted very fine sands to silts (population “c”) are trapped in the low-energy estuarine-tidal flat sub-environment, while the highly homogeneous population “b” is further sorted in aeolian dune and beach sub-environments. This sorting occurs via a coastal sand transport pathway linking the Somme estuary mouth to the southern North Sea bight where tidal range and wave energy decrease relative to the English Channel. Since this sand transport pathway enables longshore transport of hydrodynamically sorted medium to fine sand derived directly from the immediate nearshore zone, it has further contributed to a net flux of this sand population from the eastern English Channel sea bed to the southern North Sea.     

Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed

Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia.     

Mid‐ and late‐Holocene climatic changes: a test of periodicity and solar forcing in proxy‐climate data from blanket peat bogs

A wide range of palaeoenvironmental evidence from the Holocene has suggested periodicities in the Earth's climate of 10s to 1000s of years. Identifying these millennial‐, century‐ and decadal periodicities, and their impacts, is critical in developing a fuller understanding of natural climate variability. Any solar‐induced climatic change needs to be distinguished from other causes of natural climate variability and from short‐term catastrophic events induced either by external or internal processes. Such events might themselves generate a periodicity, or in combination with other forcing factors they may contribute towards a periodicity (and so spuriously imply a universal and continuing periodicity in the climate record), or they may resonate with a solar‐induced periodicity. Here, evidence from peat records for periodicity in climate change over the mid to late Holocene is reviewed and this is followed by a test of the replicability of claimed periodicities using blanket peat data covering the past 2000 yr from four sites in the British Isles. Results suggest that the mires studied do go through phases of being responsive to periodic forcing factors, with ca. 200, ca. 80 and 60–50 yr wavelengths reflected in some data sets. However, the patterns shown are not consistent. This could be the result of local conditions at individual mires (human impact, sensitivity and vegetation succession) or of changes in the strength or nature of global forcing factors. Assessing a solar–mire link remains difficult because the century‐scale variations of the Sun show different intervals between solar minima, the durations of which are themselves unequal, and because the proxy‐climate data‐sets from peat profiles may themselves not be dated with sufficient precision and/or accuracy. Copyright © 2001 John Wiley & Sons, Ltd.     

Millennial‐scale precipitation changes over Central Africa during the late Quaternary and Holocene: evidence in sediments from the Gulf of Guinea

We combine environmental magnetism, geochemical measurements and colour reflectance to study two late Quaternary sediment cores: GeoB 4905‐4 at 2° 30′ N off Cameroon and GeoB 4906‐3 at 0° 44′ N off Gabon. This area is suitable for investigating precipitation changes over Central and West Africa because of its potential to record input of aeolian and fluvial sediments. Three magnetozones representing low and high degree of alteration of the primary rock magnetic signals were identified. The magnetic signature is dominated by fine‐grained magnetite, while residual haematite prevails in the reduced intervals, showing increase in concentration and fine grain size at wet intervals. Our records also show millennial‐scale changes in climate during the last glacial and interglacial cycles. At the northern location, the past 5.5 ka are marked by high‐frequency oscillations of Ti and colour reflectance, which suggests aeolian input and hence aridity. The southern location remains under the influence of the Intertropical Convergence Zone and thus did not register aeolian signals. The millennial‐scale climatic signals indicate that drier and/or colder conditions persisted during the late Holocene and are synchronous with the 900 a climatic cycles observed in Northern Hemisphere ice core records. Copyright © 2009 John Wiley & Sons, Ltd.     

河北安固里淖孢粉记录的晚冰期以来的植被演替与气候变化

河北安固里淖地处东南季风的边缘地区,是研究古气候环境演化的理想地区之一。文章以安固里淖的沉积岩芯（长18.4 m）为研究对象,利用AMS ¹⁴C测年和孢粉数据,重建了安固里淖湖区14.3 cal.ka B.P.以来的植被与气候演化历史。结果表明：14.3~11.0 cal.ka B.P.期间,湖区以疏林草原植被为主,气候偏干;11.0~6.0 cal.ka B.P.以森林草原植被为主,气候湿润;6.0~3.2 cal.ka B.P.以疏林草原植被为主,气候偏湿;3.2~0 cal.ka B.P.以荒漠草原植被为主,气候干旱。总体而言,自晚冰期以来安固里淖的气候经历了由干变湿再逐渐变干的过程,全新世适宜期出现在早中全新世。通过区域对比发现,安固里淖揭示的晚冰期以来东南季风的气候演化特征与西南季风的演化具有很大的相似性,表明晚冰期以来东南季风与西南季风在轨道尺度上的演化并无太大差异,太阳辐射是二者演化的共同驱动力。

     

A multiproxy climate record from a raised bog in County Fermanagh,Northern Ireland: a critical examination of the link between bog surface wetness and solar variability

A proxy climate record from a raised bog in County Fermanagh, Northern Ireland, is presented. The record spans the interval between 2850 cal. yr BC and cal. yr AD 1000 and chronological control is achieved through the use of tephrochronology and ¹⁴C dating, including a wiggle‐match on one section of the record. Palaeoclimatic inferences are based on a combination of a testate amoebae‐derived water table reconstruction, peat humification and plant macrofossil analyses. This multiproxy approach enables proxy‐specific effects to be identified. Major wet shifts are registered in the proxies at ca. 1510 cal. yr BC, 750 cal. yr BC and cal. yr AD 470. Smaller magnitude shifts to wetter conditions are also recorded at ca. 380 cal. yr BC, 150 cal. yr BC, cal. yr AD 180, and cal. yr AD 690. It is hypothesised that the wet shifts are not merely local events as they appear to be linked to wider climate deteriorations in northwest Europe. Harmonic analysis of the proxies illustrates statistically significant periodicities of 580, 423–373, 307 and 265 years that may be related to wider Holocene climate cycles. This paper illustrates how the timing of climate changes registered in peat profiles records can be precisely constrained using tephrochronology to examine possible climatic responses to solar forcing. Relying on interpolated chronologies with considerable dating uncertainty must be avoided if the climatic responses to forcing mechanisms are to be fully understood. Copyright © 2007 John Wiley & Sons, Ltd.