Measurement of 10Be from Lake Malawi (Africa) Drill Core Sediments and Implications for Geochronology

Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Measurement of 10Be from Lake Malawi (Africa) drill core sediments and implications for geochronology

L.R. McHargue a,⁎, A.J.T. Jull b, A. Cohen a a Department of Geosciences, University of Arizona, 1040 E. Fourth Street, Tucson, Arizona 85721, United States b NSF Arizona AMS Laboratory, University of Arizona, 1118 E. Fourth Street, Tucson, Arizona 85721, United States article info abstract

Article history: The cosmogenic radionuclide 10Be was measured from drill core sediments from Lake Malawi in order to Received 19 August 2008 help construct a chronology for the study of the tropical paleoclimate in East Africa. Sediment samples were Received in revised form 29 January 2010 taken every 10 m from the core MAL05-1C to 80 m in depth and then from that depth in core MAL05-1B to Accepted 4 February 2010 382 m. Sediment samples were then later taken at a higher resolution of every 2 m from MAL05-1C. They Available online 12 February 2010 were then leached to remove the authigenic fraction, the leachate was processed to separate out the beryllium isotopes, and 10Be was measured at the TAMS Facility at the University of Arizona. The 10Be/9Be Keywords: ﬁ Beryllium-10 pro le from Lake Malawi sediments is similar to those derived from marine sediment cores for the late Cosmogenic radionuclide Pleistocene, and is consistent with the few radiocarbon and OSL IR measurements made from the same core. Lake sediments Nevertheless, a strong correlation between the stable isotope 9Be and the cosmogenic isotope 10Be suggests Lake Malawi that both isotopes have been well mixed before deposition unlike in some marine sediment cores. In Radiometric dating addition, the correlation of beryllium isotopes to a proxy of lake level TOC (Total Organic Matter) from Lake Malawi indicates that the concentrations of 10Be in the lake sediments result from the combined effects of global and local climates on lake level, local hydrology, and sediment transport in the Lake Malawi basin rather than as a direct response to its production in the atmosphere modulated by the intensity of the Earth's dipole. Therefore, a direct correlation of the 10Be/9Be to a chronology derived from the paleomagnetic variations measured from marine sediments was not possible. Nevertheless, a comparison of the 10Be/9Be chronology, allowing for decay, at Lake Malawi to that of the global marine paleomagnetic record suggests that the bottom of core MAL05-1B is no more than 750 ka in age. Published by Elsevier B.V.

1. Introduction (Johnson and Ng'ang'a, 1990). The lake's long, narrow and deep morphology, as well the positions of bathymetric deep areas and We investigated using the cosmogenic radionuclide 10Be to date sediment depocenters are all controlled by its origin as a series of sediments from Lake Malawi because its half life duration is linked half-graben basins (Rosendahl, 1987; Scholz and Rosendahl, appropriate for the time frame in which the lake sediments were 1990). Multiple reflection seismic surveys of the lake have shown a deposited, as well as the possibility of tying this record to that of very thick accumulation of rift sediments within the basin, in places marine sediments. More than 600m of the core have been obtained exceeding several kilometers (Scholz, 1989).Thelakeitselfis from Lake Malawi (Scholz et al., 2006, and Scholz et al., 2011-this probably at least early Pliocene in age, based on both the sediment issue), located in the southern part of the East African Rift Valley. thickness inferred from seismic data and outcrop exposures of Sediments from Lake Malawi contain an important record for the lacustrine deposits along the lake's margin (Betzler and Ring, 1995). study of the tropical paleoclimate. However, most of the sediments of Although Lake Malawi today is deep and freshwater, with an outlet the core from Lake Malawi are well beyond the approximate 50 kyr to the Shire River, many lines of evidence show that this has not range of radiocarbon dating. In addition, paleomagnetic data are always been the case. Early studies of seismic stratigraphic data from difficult to interpret because of the low latitude of the drill site Lake Malawi indicated that extraordinary lake level fluctuations had coupled with the fact that the core is unoriented. Lake Malawi lies in occurred in the lake's past (Scholz and Rosendahl, 1988), although the western branch of the African rift valley and is the second largest from that data alone the timing of such events could not be (22,490 km2) and second deepest (∼700 m) of the African rift lakes constrained. Detailed studies of some of these lake level drops of up to 550 to 600 m below modern lake level, documented by the Lake Malawi drill cores attest to extraordinary climate variability in terms ⁎ Corresponding author. Fax: +1 520 621 9619. of moisture balance (Scholz et al., 2007; Cohen et al., 2007; Brown E-mail address: [email protected] (L.R. McHargue). et al., 2007). This history is crucial in addressing questions about the

0031-0182/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.palaeo.2010.02.012 L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119 111 climate and environmental change of eastern Africa. For example, 10Be with time varies significantly. This variance in production is how have climate variables such as moisture and wind direction random in time due to the randomness in the intensity of the dipole changed over the glacial/interglacial and orbital cycles, how are these field that modulates it. Thus, a 10Be record possibly could be correlated changes tied to global climate, and how has the response of Lake to the paleomagnetic record and would be unique. The record of the Malawi to these changes influenced local and regional flora and intensity of the dipole field has been progressively pushed back to fauna? And lastly, how has this extraordinary climate variability approximately 2 Ma from stacked marine sediment core records over influenced human evolution and expansion? the last decade (Valet et al., 2005) which would cover the entire Geochronology is central to addressing these questions. The duration of sedimentation expected for the Lake Malawi drill cores. cosmogenic radionuclide 10Be has advantages that may be applicable However, variations in 10Be concentrations due to the impact of to dating the sediments of Lake Malawi. 10Be is always being produced climate on lake levels, sedimentation rate, and sediment composition in the atmosphere and removed within a year by dry and wet need to be addressed and may make the method inapplicable. precipitations. Thus, there is a constant supply of 10Be to the waters of In 2005 the Lake Malawi Drilling Project (LMDP) recovered Lake Malawi, within which the 10Be is removed to the sediments approximately 600 m of high quality drill core from two sites in below. Hence, 10Be can be measured from all the sediments Lake Malawi, the southernmost of the African Rift Valley great lakes throughout the length of the core. The presumed age of the oldest (Scholz et al., 2006,) (Fig. 1 map). Lake Malawi was drilled in order to sediments at the bottom of the core is less than the half life of 10Be obtain a high quality and continuous record of paleoclimate variability (1.34×106 yrs, Nishiizumi et al., 2007). In addition, the production of in the African continental tropics. Sedimentological indicators such as

Fig. 1. Locations of the two sites in Lake Malawi and bathymetry. Site 2, in the northern Lake Malawi is at a water depth of 361 m, and site 1 (cores B & C) is at a water depth of 593 m. 112 L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119 a dominance of sublittoral to deep water mud (in large part same sediment samples (Schwartz et al., 1998). A direct comparison laminated) and limited evidence of paleosols or other evidence of of variances best showed the relationship between 10Be and the prolonged exposure show that deposition in the core record was very paleointensity data. This was done by converting the 10Be data to continuous and, compared to similar duration marine drill core variances, and the NRM/ARM paleointensity data (Natural Remanent records, very rapid. Recent measurements of 10Be were made from Magnetization/Anhysteretic Remanent Magnetization) to inverse test samples from the recently acquired drill core from Lake Malawi, variances about linear regressions. East Africa (Scholz et al., 2006). Previous work on sediments from the Gulf of California and the The longest of the Lake Malawi cores, MAL05-1B extended to Blake Outer Ridge showed that it was possible to obtain reliable 382 m below the lake floor and a water depth of 593 m, with an cosmic-ray records from 10Be from marine sediments. Nevertheless, average core recovery of 92% (Fig. 1 core stratigraphy). Based on initial reliable isotopic and paleointensity measurements from sediments, geochronometric information (paleomagnetic reversal and excursion corals, or ice, are typically difficult to obtain. Many environmental data) this core was initially estimated to extend back to 1.5 Ma from complications affect the deposition of 10Be (Kok, 1999; Frank, 2000). estimated reversal picks, which would have made it by far the longest Lake systems are even more sensitive to short-term environmental continuous core record yet obtained from the continent of Africa. influences than the ocean due to the sensitivity of their lake levels, However, a firm chronology based on these methods alone is sediment supply, and water chemistry to regional climate. With these uncertain and other unpublished data now suggest a much younger problems in mind the earlier work on two marine sediment cores age for the base on the 1B core. Uncertainties about the interpretation from the Atlantic and the Pacific will be compared to the 10Be results of the paleomagnetic record, the temporal range limitations of both from Lake Malawi to provide context. the OSL and 14C methods, and the uncertainty of success in applying other dating methods because of limited amounts of appropriate 2. Methodology materials (e.g. Ar/Ar, and U-series dating) led us to explore how 10Be might be used for dating the older parts of the Malawi core record. The two cores obtained by the Lake Malawi Drilling Project in this The variations in the concentration of the cosmogenic radionuclide initial study were initially sampled at intervals of 10 m throughout the 10Be in marine (Robinson et al., 1995; Frank et al, 1997; Carcaillet et al., 80 m of core MAL05-1C, and in core MAL05-1B from 80 to 382 m. 2004) and lake sediments (Horiuchi et al., 1999; Ljung et al., 2007; Sediment samples were then later taken every 2 m in core MAL05-1C. Belmaker et al., 2008) can serve as a proxy for the cosmic-ray flux. The sediment samples from Lake Malawi were cut in 1-cm thick Since the intensity of the cosmic-ray flux is inversely related to the blocks, estimated to represent about an average of about 20 yrs of intensity of the geomagnetic field the history of the latter can be sedimentation. A 1–2 g split of the each sample was processed derived. The geochemical cycle of 10Be on the Earth begins as high keeping the rest of the sample in reserve. Each split was leached in a energy cosmic-rays, primarily in the upper atmosphere, break oxygen solution of 25% acetic acid and 0.04 M hydroxylamine-HCl to separate and nitrogen apart (small amounts of 10Be are also produced in situ in the authigenic mineral fraction of the sediment from the terrigenous materials on the surface of the earth and are of interest in fraction (Bourles et al., 1989). The composition of the authigenic geomorphology). Consequently, the production of 10Be is proportional fraction of the sediment more closely reflects the composition of to the cosmic-ray flux incident upon the Earth which, in turn, is a seawater than the whole sediment (McHargue et al., 2000) and is function of the intensity of the primary galactic and solar cosmic-rays, composed of exchangeable ions, carbonates, and iron–manganese and their modulation by the solar and geomagnetic fields. After 10Be is hydroyoxides. An aliquot of the leachate was diluted with 5% HNO produced it is quickly scavenged by particulate matter, and within a 3 and set aside for Inductively-Coupled Plasma Atomic Emission year is precipitated onto the surface of the earth. After deposition onto Spectrometer (ICPAES) analysis of elemental composition (Fe, Al, Ca, the surface of the earth 10Be is transported by water, wind, or currents Mn, and Mg). The residue was washed, dried, and weighed. The within the sea. Most 10Be (that which has not decayed during difference, the mass loss, between the initial mass and the residue is transport) is eventually sequestered in ice, soils, and lake and marine the presumed authigenic fraction of the sediment. A 2-mg beryllium sediments, though it may be recycled later (McHargue and Damon, carrier was added to the remaining solution. The beryllium was 1991). separated and purified by solvent extraction, precipitation of Be(OH) , Initial studies of 10Be in Antarctic ice, (Raisbeck et al., 1987) showed 2 and then combusted to BeO. that the concentration varied with depth and was found to be Beryllium-10 was measured on the Tandem Accelerator Mass anomalously high at 35 and 60 ka. These wide variations in the Spectrometer at the University of Arizona. In past measurements concentration of 10Be in time were shown to be global in extent by later the blank (10Be/9Be) for the beryllium carrier averaged less than work in marine sediments (McHargue et al., 1995). This was shown, 2×10− 14. Repetitive measurements of the NIST standard (SRM 4325) for example, at DSDP Leg 64, site 480, a drill site lying on a slope of the have determined that the typical error for a 10Be measurement with Guaymas Basin in the Gulf of California (660 m in depth) that was the tandem accelerator to be less than 2%. chosen because of its high sedimentation rate (100 cm/kyr) and varved sediments. At this site there was evidence for two geomagnetic excursions, the Mono Lake and the Laschamp, in which the inclination 3. Results and discussion of the dipole field deviated far from normal (Levi and Karlin, 1989). 10Be concentrations in sediments from the sections of the core near The sediment mass loss after leaching varied from less than 5% to these excursions were unusually high. In general, the variations in the almost 30%. A plot of the summed cation concentration for the major 10Be concentrations, except for the 10Be anomaly associated with the elements of the aliquot against the measured mass loss shows that, Mono Lake, seemed consistent with what would be expected for within the error of analytical and physical measurement of the modulation of the dipole field as shown by the research on the samples, the relationship is close to linear. Most of the weight loss is magnetization of marine and lake sediments (Tric et al., 1992; attributable to the leaching of iron and manganese, and to a lesser Meynadier et al., 1992; Thouveny et al., 1993; Carcaillet et al., 2004; extent, calcium and magnesium species. Aluminum is slightly Knudsen et al., 2008). negatively correlated to the mass loss from leaching. The concentra- From sediments from the Blake Outer Ridge (site CH88-10P) 10Be tions of beryllium in the leachate, particularly 10Be, are independent production was shown to vary inversely related to the geomagnetic of the amount of sediment loss. This may be due to the restriction of field (McHargue et al., 2000). 10Be data from this site was directly beryllium in the pore water and exchangeable phases of the sediment compared to the paleointensity data that were measured from the during accumulation of the iron/manganese hydroyoxide phases. L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119 113

We compared the data from Lake Malawi cores 1B and 1C to data dependence between 9Be and 10Be. On the other hand, in the from the Gulf of California (DSDP site 64, core 480) in order to define authigenic fraction, the concentrations of beryllium and 10Be are the differences between the depositional regimes of marine and lake largely independent at site 480 due to the different origins of 10Be and sediments. The sediments deposited in the Gulf of California are varved beryllium in the open sea. and are composed of alternating variations in the concentrations of The residue, that is the sediment remaining after leaching, was not clay from terrestrial sources and seasonally induced diatom blooms. processed for the Lake Malawi sediments for the initial study. Iron versus the aluminum obtained from the leachate from sediment Nevertheless,comparisonscanbemadebetweenthemarine samples of the Gulf of California in Fig. 2b are plotted, along with that of sediments from the Gulf of California and those from Lake Malawi. the totally digested residue after leaching. The aluminum to iron ratio There are significant differences in the composition of those of the leachate is significantly different from that of the clays and the sediments deposited in an open ocean environment to those diatoms in the residue of the varved sediments. (The silicates are not deposited in a restricted lake basin. The aluminum and iron attacked by the mild leaching during removal of the iron-enriched concentrations in the residue for the ocean sediments (Fig. 2) are authigenic fraction of the sediments.) similar to that of the terrigenous material, such as clays within the A plot of beryllium and beryllium-10 measured from the same sediments and diatoms. The aluminum to iron ratio of the authigenic samples shows that the 10Be content in the authigenic fraction of the fraction of marine sediments from the Gulf of California is positively sediment is closer to its probable oceanic content than that in the correlated and is slightly under 0.2 similar to that of sea water of 0.25 residue (Fig. 2b). The concentration of the terrigenous beryllium (9Be) (Martin and Whitfield, 1983). On the other hand, the aluminum to is higher in the clay-rich residue than in the more diatomaceous iron ratio of the authigenic fraction of Lake Malawi sediments are not residue. In the diatomaceous residue the beryllium isotopes are more correlated to one another (Fig. 3). The ratio of aluminum to iron similar to that of the authigenic fraction than the terrigenous residue ranges from 0.01 to 0.1 probably reflecting significant variations in as would be expected. Nevertheless, both the clay-rich and diatoma- lake and pore water chemistry with time affecting iron removal and ceous residues are on the same trend line indicating riverine source accumulation in the sediments.

Fig. 2. Measurements of aluminum, iron, and beryllium isotopes from two phases of the marine sediments, the leached and the residue, from the Gulf of California (DSDP Leg 64, site 480) are plotted as a comparison to the lake sediments from Lake Malawi in Fig. 4. (a) Aluminum as a function of iron from two phases of Gulf of California sediments, the leachate (authigenic fraction) and the residue (terrigenous and diatomaceous fraction). (b) 10Be as a function of total beryllium from the leachate and residue of marine sediments from the Gulf of California. 114 L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119

Fig. 3. Measurements of aluminum, iron, and beryllium isotopes from Lake Malawi sediments from the authigenic fraction only. The scale for Fig. 4a and b has been kept the same as that in Fig. 3 for comparison to each other. (a) Aluminum as a function of iron from the authigenic fraction of the sediments from Lake Malawi. Note that the vertical scale is 1/25 that of Fig. 3a. (b) 10Be as a function of beryllium from the authigenic fraction of the sediments from Lake Malawi. R is the correlation coefﬁcient. Note that the vertical scale is 1/4 that of Fig. 3b.

There is a significant difference between the concentrations of the 480 directly related to its production in the atmosphere. On the other beryllium isotopes between the authigenic fraction of the marine and hand, 10Be and 9Be in the sediments at Lake Malawi are more strongly lake sediments. The concentration of 10Be to beryllium is 50 to 80 correlated (R=0.80) than the marine sediments from the Gulf of times higher in the marine sediments of the Gulf of California than in California. A significant amount of the 10Be deposited in the sediments the lake sediments due to increased scavenging from vigorous at Lake Malawi has been recycled and mixed with 9Be. The variations upwelling. The residence time of 10Be in the open sea is nearly in the iron and aluminum data and the dependence of 10Be on 9Be 1000 yrs (McHargue and Damon, 1991), whereas that in the lake may indicate that the interpretation of the 10Be data from Lake Malawi as a be in the order of a few yrs to a few decades depending on the river representative primarily of cosmogenic deposition may be problem- input, lake levels, and sediment deposition. atical. The extent to which 10Be that is produced in the atmosphere More importantly the distribution of 10Be in the authigenic just prior to precipitation to Lake Malawi can be distinguished from fraction with respect to 9Be is significantly different. At DSDP site that recycled through aeolian and hydrological processes is therefore 480 10Be is not correlated to beryllium (r=0). This is as a result of uncertain. 10Be and beryllium ultimately having different sources and not having Fig. 4 shows the two isotopes of beryllium plotted separately for mixed before their removal to the sediments. Almost all 10Be both core C and B from the Lake Malawi sediments. As the strong originates in the atmosphere (Lal, 1988). Only a small amount of correlation in Fig. 3b indicates there are similarities between the two 10Be (b1%) is produced in situ, that is, within the surface materials of plots of beryllium (Fig. 4a) and 10Be (Fig. 4b). Most notably at a depth the earth. Beryllium (9Be), the stable form of the isotope, instead is of 150 m there is a sharp increase in both beryllium and 10Be probably largely derived from terrigenous materials. The two isotopes mix due to a sharp decrease in the sedimentation rate. In addition, both during transport in the hydrological cycle. At site 480 the complete show a trend to lesser concentrations at depth in the core, beryllium independence of 10Be from 9Be indicates little to no mixing of the two weakly (r=−0.39) and 10Be more strongly (r=−0.62), perhaps due isotopes before deposition. Thus the deposition of 10Be at site DSDP to decay. Mixing of both isotopes in the watershed of the lake during L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119 115

Fig. 4. Beryllium isotopes as a function of depth cores 1C and 1B together for the sediments of Lake Malawi. (a) The concentration of beryllium (ppm) from the authigenic fraction of the sediments as a function of depth. The sampling changed from approximately every 2m at depths shallower than 80 m to 10 m or more for the depth interval from 80 to 370 m. (b) The concentration of 10Be (in atoms/g) from the authigenic fraction from the same sediment samples as in Fig. 5a.

soil formation, riverine and aeolian transportation of sediments, and For comparison to the Lake Malawi data the global production of deep water deposition would be expected. 10Be as a function of the averaged paleomagnetic field intensities from A higher resolution look of the trends for 10Be and beryllium for several marine sediment cores is shown in Fig. 6b(Valet et al., 2005). core 1C is shown in Fig. 5a and b, respectively. Also shown in Fig. 5c In Fig. 6a the concentration of 10Be/9Be in the core C at site 1 at Lake are the TOC (Total Organic Carbon) data for the same section for Malawi core is plotted with depth. From the organic matter from the comparison (Lyons et al., 2011-this issue). The TOC is a proxy for the upper most part of the core numerous radiocarbon dates are available past lake depth of Lake Malawi (Lyons et al., 2011-this issue). During and the depth and time boundaries are plotted (Scholz et al., 2007, lowstands of the lake the TOC is low when arid conditions (Cohen et Supporting Information). These depths of the youngest and oldest al., 2007) limit runoff and sediment to coarser grained inputs. During radiocarbon-dated samples in Fig. 6a are correlated with their dates in highstands, along with the increased sediment supply in more finely Fig. 6b. In addition, the oldest obtained OSL IR (Optically-stimulated laminated intervals, there is an increase in the terrestrial organic luminescence dating) result is correlated in the same manner thus matter (Cohen et al., 2007). The depth intervals for transgressions bracketing the sediments measured in time. Also shown for and regressions of the lake level in Fig. 5c are highlighted by the comparison in Fig. 6b are the MIS (Marine Isotopic Stages) associated dotted lines and correlated to the beryllium isotope data in Fig. 5a with this time period. The increases in the 10Be/9Be ratio are roughly and b. The concentration of the beryllium isotopes do not directly consistent with the 10Be production rates as derived from the correlate with the concentration of the organic matter in the paleomagnetic intensity record. The 10Be/9Be ratio is low in the sediments. However, the regressions and transgressions of Lake Holocene due to the more intense dipole fields. The maximum 10Be/ Malawi as reflected by the TOC data is also, in part, reflected in the 9Be in MIS 3 may be associated with the Laschamp geomagnetic beryllium isotope data. For example, the lowering of the lake level excursion. A secondary peak of 10Be in MIS 4 is consistent with similar between 45 and 70 m is associated with a decrease in the increases in 10Be concentrations seen in marine sediment cores and concentration of the beryllium isotopes over the same section. In ice cores in Antarctica. A significant 10Be/9Be increase, and associated addition, a rise in the lake level interpreted from the sediments perhaps with the Blake Event, is more sharply defined in the sediments between 85 and 70 m corresponds to an increase in the concentration than would be expected from the 10Be production curve but is of the beryllium isotopes. Similarly, a rise in the lake level from 30 to consistent with the 10Be data from the Blake Outer Ridge (not shown). 10 m, followed by a lowering for sediments less than 10 m of core The above interpretation assumes the excesses of 10Be to 9Be in the depth, correspond to an increase and then a decrease in beryllium 10Be/9Be ratio are due to increases in the production rate of 10Be in the isotopes, respectively. On the other hand, a significant transgression atmosphere. However, the excess of 10Be to 9Be may result from an between 38 m and 30 m is not associated with an increase in increased collection for 10Be precipitating from the atmosphere due to beryllium isotopes but a decrease. a greatly expanded surface area of Lake Malawi during highstands 116 L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119

Fig. 5. Beryllium isotopes and TOC for core 1C from late Pleistocene Lake Malawi sediments. (a) The concentration of 10Be from the authigenic fraction of the sediments as a function of depth. (b) The concentration of beryllium from the authigenic fraction of the sediments as a function of depth. (c) The concentration of organic matter (TOC) from the sediments as a function of depth (Lyons et al. 2011-this issue). Major long-term changes in the TOC are correlated with the beryllium isotopes by vertical dashed lines. Declines in lake level correspond to decreases in TOC and lake level rises that of increases in TOC.

with respect to recycled 10Be and 9Be. Therefore, the above correla- against the plot of the geomagnetic field. The dotted lines show tions are problematical. possible correlations to broad features both in the 10Be/9Be data and The history of the dipole field has been extended to 2million yrs by the geomagnetic data. There does seem to be a rough similarity stacking together the data from sediment cores from different areas of between the two data sets. For example, a significant decrease in 10Be/ the world (Valet et al., 2005). In Fig. 7 the averaged intensity of the 9Be at depths in the core deeper than 300 m and an increase from 200 geomagnetic field by Valet et al (2005) (Fig. 7b) is compared to to 300 m is reflected in the 10Be production curve for the time interval the 10Be/9Be ratio for the entire core 1B from Lake Malawi (Fig. 7a). older than 400 kyr. Also, the rate of the decrease in 10Be/9Be is The mean paleomagnetic dipole field intensities (Valet et al., 2005) consistent with the decay of 10Be. The implication from Fig. 7 is that have been converted to the production of 10Be by the method of Lal the bottom of the core may be between 650 to 750thousand yrs in (1988). In order to make comparisons of the 10Be data as measured age. from the sediments in terms of concentration (atoms/g), or by the ratio However, as was discussed earlier, it is not likely that the 10Be of 10Be to beryllium (10Be/9Be), it is necessary to take in account the concentrations in the Lake Malawi sediments are directly tied to its decay of 10Be (half life of 1.34million yrs). The concentrations of 10Be production in the atmosphere but is mostly recycled from several have not been corrected to their “original” concentrations because the sources. In addition, the changes in lake level affect the transport of time scale has not been established. Therefore, the production of 10Be the beryllium isotopes to the lake, and the changing surface area of the has been modified to reflect the decay since those earlier periods of lake effects the collection of atmospheric 10Be. The correlation time — the production of 10Be has not increased with time as might be between the two data sets may be either coincidence, or a secondary inferred from Fig. 6. The sediments below 80 m were taken every 10 m correlation between climate and the geomagnetic field and then so all the results down to 380 m have been compressed for comparison between climate, lake level, and sediment deposition. L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119 117

Fig. 6. The ratio of beryllium isotopes from the late Pleistocene Lake Malawi sediments, core 1C, are correlated to an averaged 10Be production curve derived from marine sediments. (a) 10Be/9Be as a function of depth for Lake Malawi sediments from core 1C, site 1. Two dashed lines bracket the depths of the range of C-14 dates, and one dashed line corresponds to the depth for the oldest OSL IR date. (b) 10Be averaged global production (Valet et al., 2005) with possible correlation ties as dotted lines to Fig. 5a. Marine Isotope Stages (MIS) 1 through 6 are shown.

4. Conclusion more importantly the wide variations in the lake level affect the flux of the beryllium isotopes to the lake sediments significantly. When Measurements of 10Be were made on the Lake Malawi drill core the lake level is high there is an increased flux of beryllium isotopes to MAL05-1C at intervals of every 2 m to a depth of 80 m and then every the lake from the surrounding drainage basin. In addition, the 10 m from core 1B to near the bottom at 380 m. 10Be and 9Be, in increased surface area of the lake enhances the collection of 10Be addition to several major elements, indicate that a record of past lake from the atmosphere with respect to that of the recycled beryllium water chemistry can be obtained from the authigenic fraction of the isotopes. When the lake level is low during times of aridity transport sediments. Comparisons between the results for 10Be/9Be and those of the beryllium isotopes to the lake from the local drainage basin is from a marine sediment core from the Gulf of California highlighted restricted and direct precipitation of 10Be from the atmosphere is less some differences between the oceanic and lacustrine sedimentary due to a much smaller lake surface area. Thus, any correspondence regimes. A profile for the 10Be/9Be ratio from Lake Malawi does show between the 10Be/9Be profile and the known 10Be production profile correspondence, with some differences, with the global 10Be produc- may reflect an indirect relationship between the dipole field and tion for the late Quaternary. The bottom of the core is no older in age climate. The effects of climate on lake levels, sedimentation rate, and than 750,000 yrs (perhaps as young as 650,000 yrs) rather than the water chemistry will affect the concentration of 10Be within the 1.5million yrs of original estimates. However, the correlation between sediments, and climate, in part, may be influenced by the dipole field. 10Be and 9Be in Lake Malawi sediments unlike that of the Gulf of Unfortunately, the effects on the local climate on sedimentation and California suggests a significant recycled component of 10Be. Perhaps lake levels may obscure even the effects of the secondary correlation, 118 L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119

Fig. 7. Beryllium isotopes from both cores 1C and 1B are correlated to an averaged 10Be production curve derived from Pleistocene marine sediments. (a) 10Be/9Be for Lake Malawi sediments for cores 1C and 1B with dotted lines as possible correlations to Fig. 6b. (b) 10Be averaged Quaternary global production from Valet et al. (2005) modiﬁed by the decay of 10Be.

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