Sediment Cores from the Kalya platform, slope and horst: High resolution and long-time span paleolimnology and paleoclimate records

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1 Sediment Cores from the Kalya platform, slope and horst: High resolution and long-time span paleolimnology and paleoclimate records Students: Derrick Zilifi and Meagan Eagle Mentor: Dr. Kiram Lezzar Introduction The large lakes of the East African rift valley are underlain with thick sedimentary sequences containing significant paleoclimate and paleolimnological records (Pilskaln & Johnson, 1991). Lake Tanganyika is the largest (32,600 km 2 ), deepest (1470 m) and oldest (>12 Ma) of the East African Rift Lakes (Cohen et al, 1993), so potentially contains very long time-scale, high-resolution sediment records. Previous studies of sediment cores from Lake Tanganyika correlate changes in sedimentation with shifting limnological and climatic regimes over the broad region of equatorial Southern Africa where Lake Tanganyika is situated (Cohen et al, 1997; Tiercelin et al, 1992). Preliminary analysis of the stratigraphy, organic and inorganic carbon (OC and IC) of four cores obtained from the platform, slope and horst of Kalya in the southern basin of Lake Tanganyika (Fig 1, Henderson & Gans, Nyanza Project Annual Report 2000) offers a new site for high-resolution continuous sediment records in Lake Tanganyika. Selection of the study area The Kalya Horst is a structural high that separate two subsiding half-grabens at the northern tip of the Southern Lake Tanganyika Rift Basin. This horst is a northwest trending ridge bounded by NNW trending border faults with opposite dip. The basinal bedrock lithology consists of Precambrian pre-rift lacustrine sedimentary rocks. Previous studies have collected deep multichannel lines (Rosendahl, 1987), but no studies have yet examined in detail that horst s sediments. The location of the Kalya Horst is ideal for obtaining long, continuous sequences because it is a structural high, located 20 km from shore and removed from the effects of terrestrial sediment input. The oxicline for the southern basin currently fluctuates between 130 m in the dry season and 100 m in the rainy season, leaving the horst core sites in perpetually anoxic conditions, perfect for the preservation of organic matter. Four cores were collected from the Kalya region in a transect across the platform (LT00-01 at 128 m water depth), slope (LT00-02 at 309 m) and the horst (LT00-03 at 608 m and LT00-04 at 578 m). Methodology Four gravity cores (from 115 to 172 cm in length) were collected using a crane and winch from aboard the M/V Maman Benita on a 6 days Nyanza Project cruise during July The cores were brought to the Nyanza Project research facilities at TAFIRI (Kigoma, Tanzania) and stored vertically prior to opening to preserve stratigraphy. Stratigraphic columns were drawn immediately after the core was opened and colors and structures were described, and then photographed for future analysis. The sediments in the cores were analyzed under a microscope (10x64) to refine facies descriptions, including the dominant mineralogy, grain size, and microfossil content. The cores were subsampled every 5 cm (cores LT00-02 and LT00-03) or every 10 cm (cores LT00-01 and LT00-04) for total organic carbon (OC) and inorganic carbon (IC). OC and IC were determined using loss on ignition (LOI) methods with a muffle furnace. OC is found by firing the sample to 550 C and applying the following equation: (Pre-fired sample weight 550 Fired sample weight) x 100 x *R OC = % OC Beginning sample weight (550 Fired sample weight 925 Fired sample weight) x 100 x *R IC = % IC Beginning sample weight * The correction factors applied to these equations, R OC and R IC, are based on the percent carbon in the organic matter and the percent carbon in the carbonate fraction. Using CH 2 O as typical organic matter, R OC

2 Figure 1 Burundi Kigoma Tanzania D.R. Congo Study Site Zambia m 6.42 Lugonesi River Msofwe River K4 K5 Sibwesa Village K6 MAHALE MOUNTAINS Mpanga River Cultivation Kalya Village Kisinsa River Cultivation LT00-01 WD 128m S A N D Y S H O R E L I N E Swamp K2 K3 Lufubu River K1 Southern Extension of the Mahale Mountains Steep Escarpment 1200m Rocky Shoreline Central Lake Tanganyika LT WD 309m LT00-04 WD 578m N 6.67 LT00-03 WD 608m km

3 is 0.40; and using CaCO 3 as a typical carbonate, R IC is 0.12 (G. Ellis & C.A. Scholz., personal communications). The species used for correction factors are typical species, used in the absence of known organic matter and carbonate species for the LT00 cores. Results Detailed stratigraphic analysis on cores LT00-01, LT00-02, LT00-03 and LT00-04 provides a highresolution sediment record. Major facies were identified and correlated between each core for paleoenvironment interpretations, and correlated with OC and IC plots for each core. All descriptions work upcore (Fig. 2 & 3). LT00-01 (128m water depth, and 115 cm length) has four main lithological facies (Fig. 4). The layered clay of Facies 1 ( cm) has 0.25 cm thick light and dark olive gray clay layers with low OC and IC (4.17% and 0.29%) compared to the slope and horst cores, as expected due to oxic conditions (Fig. 3). The fecal pellet clay of Facies 2 ( cm) is dominated by alternating layers of fecal pellet rich clays in sharp contact with light gray massive clay layers 1-3 cm thick. Massive clay layers on the shallow platform represent density current deposits and indicate that the sedimentation on the platform is active today. Facies 3 (46-73 cm) is a diatom-rich clay, with yellow-gray diatom rich layers alternating with gray layers every centimeter. OC and IC remain steady for facies 2 and 3 (5.25% and 0.33%). Facies 4 (0-46 cm) is a layered clay that contains alternating layers of fecal pellet and massive clays, similar to facies 2. A sand lens at cm embedded in a clay unit most likely represents slumping on the platform and is accompanied by a decrease in OC and IC (4.88% and 0.28%) due to terrestrial dilution. LT00-02 has three lithological facies (Fig. 5). The gray silt of Facies 1 ( cm) is a silty diatomaceous dark olive gray unit with a few diatom ooze laminations. OC (6.31%) is reduced from the laminated unit up-core (6.78%). Lake productivity during the deposition of this unit may be lower than the above units, or increased autochthonous sedimentation may be diluting OC. Facies 2 ( cm) is a flocculated clay that alternates between yellow-gray flocculated diatom-rich and dark gray clay laminated bundles. Facies 3 (0-46 cm) is a diatom-rich flocculated clay with yellow-gray flocculated layers larger than the previous unit, dominating the clay rich bundles. This facies change is possibly a result of a recent increase in lake productivity. The flocculated units are cohesive and large (1-5 cm across) and full of benthic diatoms (Michelo, V. Nyanza Project Annual Report 2000). These flocculated units may aggregate while the diatoms are alive and then the entire unit is transported away from the platform. Turbidity and dissolved inorganic carbon are high in the southern basin from m, which may indicate the presence of particulate rich currents within the water column (Adams & Charles, Nyanza Project Report 2000). LT00-02 is laminated over 91% of the core length, with light and dark laminae making up a 1 mm thick couplets, providing a sediment record with seasonal resolution. OC levels are reduced compared to Scholz et al. (Kavala Island Ridge, in review at Paleo3) Core T97-52V s 10-12% OC at comparable water depths (LT00-02 at 309 m and T97-52V at 393 m). The OC and IC graphs (Fig 5) display an interesting cylicity with evenly spaced peaks throughout the core length. This variation may be due to periodic changes in lake conditions. LT00-03 (608 m, 170 cm) (Fig. 6). The basal clay of Facies 1 ( cm) contains alternating laminated layers of yellow-gray diatomaceous ooze with a dark gray clay rich in terrestrial plant fragments. Stephanodiscus dominates the diatom rich laminated layers and Aulacoseira, in lesser abundance, dominates the gray layers (for complete discussion of diatom assemblages in core LT00-02 and LT00-03, refer Michelo, V. (Nyanza Project Annual, Report 2000). OC and IC fluctuate throughout facies 1, with the average values for OC high (7.92%). IC (0.33%) fluctuates synchronously with the OC. Facies 2 ( cm) gray clay is non-laminated with small ( mm) flocculated clay particles and Stephanodiscus appears in low abundance. The banded clay of Facies 3 (59-88 cm) is also non-laminated with several prominent dark brown and light gray clay bundles.

4 Figure 2: Correlations between LT00 Cores based on facies relationships. LT m water depth 0 cm Layered Diatom LT m water depth 0 cm Fecal Pellet Layered Diatom Diatomrich Flocculated Flocculated 0 cm LT m water depth Laminated Diatomaceous Ooze Gray LT m water depth 0 cm Laminated Diatomaceous Ooze Laminated Gray 115 cm Layerd Banded LT cm (wood fragment) 2845+/-48 C-14 yr. LT (bulk organic) 3303+/-40 C-14 yr Basal Gray Silt 172 cm Gray LT (bulk organic) 9280+/-55 C-14 yr 117 cm Basal 170cm

5 Figure 3 LT00 Cores % TOC (LOI corrected) 0 %TOC 0% 5% 10% 15% Length LT00-01 LT00-03 LT00-02 LT00-04

6 Figure %IC & % OC LOI corrected 0% 5% 10% 15% LT00-01 S ' E ' 128 m Facies 4 (0-46 cm) contains alternating layers of fecal pellet rich clays in sharp contact with massive clays. Laminations occur in several of the clay units. Sand lens at cm is embedded in a clay unit. 40 Length 60 Facies 3 (46-73 cm) is a laminated clay unit, with yellow-gray diatom ooze laminations alternating with gray clay Facies 2 ( cm) has alternating layers of fecal pellet rich clays in sharp contact with light gray massive clay layers 1-3 cm thick. Facies 1 ( cm) is layered (0.25 cm thick) with alternating light and dark olive gray clay. 120 TOC% Carbonate

7 The laminated clay of Facies 4 (50-59 cm) is a dark gray and medium gray clay unit that grades into Facies 5 (37-50 cm), a massive gray clay unit with sparse terrestrial plant fragments. OC (6.5%) and IC (0.28%) remain steady throughout facies 2, 3, 4 and 5, and are reduced from the OC and IC levels in the laminated units, indicating a less productive lake regime. The laminated diatom ooze of Facies 6 (0-37 cm) resumes distinct laminations between yellow-gray diatom rich ooze and dark gray clay bundles, and is has increased levels of OC (9.33%) and IC (0.42%). LT00-04 (578 m, 117 cm) has three dominant facies (Fig. 7). The basal clay of Facies 1 ( cm) contains alternating light and dark gray bundles of laminated gray diatom ooze and dark gray clay. Stephanodiscus again dominates the diatom-rich laminations. Dark gray bundles within the laminated units signal terrigenous input during the deposition of the diatom ooze. Similar to LT00-03, OC (9.32%) and IC (0.42%) increase in the laminated units. Facies 2 (50-64 cm) is a medium gray, faintly laminated, clay. Facies 3 (0-50 cm) is a laminated diatom ooze with yellow-gray 100% diatom rich laminae alternating with brownish black and light medium gray laminae couplets. OC and IC (6.75% and 0.36%) remain stable throughout facies 2 and 3. Discussion Lake Tanganyika is currently in a productive, well-stratified, high lake stand and under the current sediment regime organic-rich, laminated units are deposited on the slope and horst. We interpreted the down-core laminated, OC-rich units (Laminated Diatom Ooze and Flocculated ) to represent highly productive lake conditions with excellent preservation of organic material in the anoxic bottom waters. During cold, dry climates associated with lower lake stands, sedimentation is dominated by clay facies (Basal and Gray ) seen in the horst cores. Lake level curves (Fig. 6 & 7) use changing sedimentation conditions on the horst as a proxy for lake level changes. The tops of the two horst cores can be correlated through stratigraphy and geochemistry. Both have Laminated Diatom Ooze in the top units and show similar enrichments in OC and IC. Below this correlated unit in LT00-03 are a series of massive and layered clays, covering 72 cm, while LT00-04 only has a 14 cm thick clay unit. The basal clays of both cores are well laminated, diatom ooze and clay, and appear to be the same layer. This suggests a lower sedimentation rate in the top unit of LT00-03 compared to LT00-04, but the Gray units of LT00-03 probably had higher sedimentation rates than LT LT00-03 is down slope from LT00-04, so when the location of the LT00-03 is subsiding, sedimentation rates increase. The observed differences in sedimentation rates on the horst may be attributed to neotectonic readjustments of intra-horst faults and a reduced subsidence of the horst arm. Following previous studies, lake level reconstructions based on stratigraphy and organic matter is possible (Pilskaln et al, 1991). The horst and slope cores contain high levels of organic matter (OC) (slope average 6.89%, max 9.71%; and horst average 7.80%, max 11.82%) compared to values published for the northern Bujumbura sub-basin (average 4.3% and maximum 12.0%) (Tiercelin et al, 1992), and slightly reduced from the nearby Kavala Island Ridge (average 10-12% and maximum 14 %) (Scholz et al., in review). LT00-02 is continuously laminated for the entire length of the core, so using published sedimentation rates from similar environments of 1mm/yr in Lake Tanganyika, the core covers 1700 years. The similarity of flocculated units throughout the core suggests that productivity has remained high for this time span. Similar paleoproductivity analysis can be done on the horst cores. The Basal and Laminated Diatom Ooze of LT00-03 and LT00-04 represents highly productive regimes similar to today s conditions, and three thick massive Gray units represent low lake stands when the lake was not as productive. Preliminary lake level reconstructions from LT00-03 and LT00-04 show slight reductions in productivity (decreased OC) during low lake stands (Fig. 6 and 7). Using Scholz et al (in review) sedimentation rates of 0.1 mm/yr (from the Kavala Island Ridge, a similar environment to the Kalya Horst but deeper) LT00-03 may span 17,000 years of sedimentation. With these sedimentation rates, low lake stands dominated from 3,700 to 5,000 yr BP and 8,800 to 10,900 yr B.P. These last dates may coincide with the low lake stands of the Younger Dryas recorded in nearby Lakes Natron and Magadi at 10,000 to 11,000 yr BP (Roberts et al, 1993).

8 Figure % IC & % OC LOI corrected 0% 5% 10% 15% 2 1 LT00-02 S ' E ' 309 m Facies 3 (0-46 cm) has thick yellow-gray diatom-rich flocculated clay layers Facies 2 ( cm) alternaes betwen yellow-gray flocculated diatom-rich layers and dark gray clay laminated layers. The flocculated units are cohesive and large (1-5 cm across) and full of benthic diatoms 80 Length cm (wood fragment) 2845+/-48 C-14 yr (bulk organic) 3303+/-40 C-14 yr Facies 1 ( cm) is a silty diatomaceous dark olive gray unit with a few diatom ooze laminations. TOC% Carbonate

9 Figure 6 % IC & % OC LOI corrected 0% 5% 10% 15% 0 20 LT00-03 S ' E ' 608 m Lake Level (Present = +) Facies 6 (0-37 cm) is a brownish black clay with distinct yellow-gray diatom rich laminae and dark gray laminae containing terrigenous plant material 40 Facies 5 (37-50 cm) is a massive gray clay unit with sparse terrestrial plant fragments grading into Facies 4 (50-59 cm) dark and medium gray laminated clay. Length Facies 3 (59-88 cm) is a medium dark gray massive clay. Dark gray clay (69 cm) Yellow gray clay (77 cm) - Dark brown clay (87 cm). - Facies 2 ( cm) is dominated by a non-laminated, dark gray clay. There are small ( mm) floculated clay particles and Stephanodiscus appears in low abundance cm (bulk organic) 9280+/-55 C-14 yr Silty diatomaceous layer at 125 cm shows a reduced TOC. Facies 1 ( cm) has mm-scale laminated yellow-gray diatomaceous ooze alternating with a dark gray clay rich in terrestrial plant fragments. Stephanodiscus diatoms dominate the yellow-gray laminations and Aulocaseria, in lesser abundance, dominates the gray laminations TOC% Carbonate

10 Figure 7 % IC & % OC LOI corrected 0% 10% 0 LT00-04 S ' E ' 578 m Facies 3 (0-50 cm) has three distinct laminations. The yellow-gray 100% diatom ooze laminations alternate with brownish black and light medium gray laminations Facies 2 (50-64 cm) is a medium gray faintly laminated clay. Length Facies 1 ( cm) contains alternating layer of light and dark gry laminted clay and yellow-gray diatom-rich ooze laminated with dark gray laminated clay. Stephanodiscus dominates the diatom-rich laminations. 120 TOC% Carbonate

11 Conclusion Analysis of the Kalya cores shows that sediments in the slope and horst region are rich in OC and IC and preserve seasonal signals at varying resolutions, thus providing an excellent opportunity for paleolimnological and paleoclimate reconstructions. LT00-02 is continuously laminated with seasonal varves for nearly the entire length of the core, while LT00-03 and LT00-04 have very fine laminations that can provide long-scale annual resolution records. The differing resolution of slope and horst cores offers an exciting opportunity to correlate between the longer time range of the horst and the high-resolution slope cores. Recent Research Progress as of February 2001 AMS 14 C dates on wood fragment and bulk organic from cores LT and LT have been recently provided (January 2001) by the AMS Laboratory at the University of Arizona. LT cm (wood fragment) 2845+/-48 C-14 yr. LT (bulk organic) 3303+/-40 C-14 yr LT (bulk organic) 9280+/-55 C-14 yr These dates will be included and discussed in a paper we are working on (February 2001) with Luc Andre, Damien Cardinal, Andy Cohen, Geoffrey Ellis and Kiram Lezzar. Further Research An intensive coring expedition to the Kalya Horst would add exciting and important paleolimnological and paleoclimate records for the East African Lakes region. Complete analysis of organic matter and carbonate species, isotope analysis, more C14 dating, fine grain size analysis, and further investigation of diatom assemblages are necessary to constrain the age of the core and provide sedimentation rates for the Kalya region in central Lake Tanganyika. Acknowledgements Our mentor, Dr. Kiram Lezzar, provided endless support and encouragement throughout the entire project. He guided us through an exciting analysis of all cores collected. David Knox was a great help with technical details and Kamina Chororoka provided the hamna shida attitude. We thank the AMS Laboratory at the University of Arizona for the 14 C dating. The crew of the M/V Maman Benita provided a wonderful coring team in the windy Southern Basin of Lake Tanganyika. Above all we would like to thank the Nyanza Project for the opportunity to study Lake Tanganyika. References Cohen, A.S., Soreghan. M. & Scholz, C.A. (1993) Estimating the age of rift lake basin. Geology, V. 36 (3): Cohen, A.S., Lezzar., K.E. Tiercelin, J.-J. and M. Soreghan (1997) New Palaeogeographic reconstructions of Lake Tanganyika: Implications for tectonic, climatic and biologic evolution in a rift lake. Basin Research., 9, 3, p Ellis, G. & Scholz, C.A. (Personal Communications). RSMAS, University of Miami, FL and Lacustrine Rift Basin Program, Department of Earth Sciences, Syracuse University, NY. Pilskaln, C.H. & Johnson, T.C. (1991) "Seasonal Signals in Lake Malawi Sediments," Limnology & Oceanography, V. 36 (3): Roberts, N., Taieb, M., Barker, P., Damanti, B, Icole, M & Williamson, D. (1993) Timing of the Younger Dryas event in East Africa from lake-level changes, Nature V. 366, Nov Rosendahl, B. (1987) Architecture of Continental Rifts with Special Reference to East Africa. Annual Review Earth Planet Science V. 15: Scholz, C.A., King, J.W., Ellis, G.S., Swart P.K., Stager, J.C., & Colman S.M. (in review) Paleolimnology of Lake Tanganyika, East Africa, over the past 100 kyr, for Special Issue of Paleolimnology Keynote papers from LENNOU, 2 nd Congress on Limnogeology Tiercelin, J., Soreghan, M., Cohen, A.S., Lezzar, K. & Bouroullec, J. (1992) "Sedimentation in Large Rift Lakes: Example from the Middle Pleistocene--Modern Deposits of the Tanganyika Trough, East African Rift System," Elf Aquitaine Production (BCREDP 16).