Elemental and isotopic compositions of organic carbon and nitrogen of recently deposited organic matter in Empakai crater and its implication for climatic changes in northern Tanzania

A 29 cm long core recovered from a water depth of 5 m in a small closed lake located in the Empakai crater northern Tanzania, is used to document the contents of organic carbon and nitrogen, stable isotopes composition of organic carbon and nitrogen, and C/N ratios and to infer climatic changes from these data. The &#948 13C and &#94815N values for this core, which averages -16.5±2.1l and 11.0±5.6l respectively, show a general downcore decrease below 5 cm. However, within this general trend, two peaks enriched in 13C are observable at about 8 cm and 13 cm. The peak at about 13 cm corresponds also to a zone depleted in 15N values. The general downcore trend can be attributed to systematic changes in relative proportion between C3 and C4 types of organic matter (OM), resulting from climatic changes or nutrient changes and shift between algae and higher plants. The lower most section containing the most depleted values can be attributed either to high input of autochthonous material owing to wetter/humid conditions or high contribution of C3 high land plant materials in response to increase in forest cover. The general downcore decrease in 13C values can also be interpreted to indicate that precipitation and vegetation cover in the area has been declining over time, and occasionally drought has been intense particularly during two zones that are characterized by enrichment in 13C. The upper 5 cm is characterized by near constant values of 13C and 15N with isotope values as high as -13.15‰ and 22.15‰ respectively. High enrichment in 15N in the upper 5 cm could be attributed to denitrification and ammonium volatilization processes, while enrichment in 13C in the upper 5 cm could be attributed to high proportion of C4 type of OM and/or utilization of HCO3- by phytoplankton during synthesis of OM. This site is also characterized by high concentrations of organic carbon (averaging 7.8±2.0%) and nitrogen (averaging 0.8±0.3%). A smell of hydrogen sulfide detected in water samples collected at approximately 20 m, suggest that there is enhanced preservation of OM in the lake under anoxic condition. The C/N ratios values, which average 12.2±2.3, increase downcore to the base of the core owing to preferential loss of nitrogen containing compounds during diagenesis. Tanzania Journal of Science Vol. 30 (2) 2004: pp. 87-9

Stable isotope compositions of organic carbon and contents of organic carbon and nitrogen of lacustrine sediments from sub-arid northern Tanzania

The stable isotope compositions of organic carbon (OC), and contents of OC and nitrogen for four sediment cores recovered from lakes Makat (located in the Ngorongoro Crater), Ndutu and Masek (located in the Serengeti Plains) are used to document sources of organic matter (OM) and climatic changes in sub-arid northern Tanzania during the late Pleistocene-Holocene period. Accelerate mass spectrometer (AMS) 14C ages on total OM for sediments collected from the Ngorongoro Crater Lake indicate that the sedimentation rate is approximately 17 cm/ka. The &#94813C values from the 20 cm long core (short core) show a downcore increase, whereas that of 500 cm long core (long core), show two peaks enriched in 13C and three peaks depleted in 13C. A general downcore increase in the 13C values for the short core suggests changes in the relative proportion of C3 and C4 with C4 fraction increasing downcore. Similarly, low and high peaks in the long core suggest changes in the relative proportion of C3 and C4 with low values having high proportion of C3 type of material, probably indicating changes in precipitation and lake levels in the area. Deposition of OM depleted in 13C took place during periods of high precipitation and high lake levels. Although high content of OC and nitrogen in some core sections are associated with elevated C/N ratio values, diagenetic alteration of isotope signature is unlikely to have caused OC isotope enrichment in sections having high contents of OC and nitrogen. The OC isotope record from Lake Ndutu shows a general downcore decrease in &#94813C values and contents of OC and nitrogen. Furthermore, this general trend is punctuated by a period of above normal enrichment in 13C, with &#94813C values being as high as -6‰. A downcore decrease in &#94813C values and contents of OC and nitrogen could be attributed either to climatic variability or diagenetic effect. Moreover, a peak enriched in 13C probably indicates a period of reduced precipitation, where phytoplankton used dissolved bicarbonate rather than dissolved atmospheric CO2. Lake Mesak record shows two peaks depleted in 13C that are flanked by near constant values of about -20‰. These two peaks (depleted in 13C), which most likely have high proportion of C3 type of OM, are associated with low contents of OC and nitrogen. Generally, the contents of OC and nitrogen for this lake are lowest relative to other lakes. Tanzania Journal of Science Vol. 32 (1) 2006: pp. 61-7

Recent trends in accumulation rate, elemental and isotopic composition of organic matter in the Magu bay, Lake Victoria

Stable isotope composition of organic carbon (OC), 14C data, OC and nitrogen content of recently deposited sediments in Magu Bay of Speke Gulf, Lake Victoria are used to document accumulation rates, sources and distribution of organic matter in the bay. The stable isotope values of both organic carbon and nitrogen decrease towards offshore, and this trend is associated with increase in the contents of organic carbon and nitrogen. The isotope results show that the organic material deposited in the bay are generally derived from C4 type of plants and are not transported far from the river mouth. Also the results show that preservation of organic matter is high in open water probably due to high primary productivity owing to high concentration of nutrients and low in turbidity. Low isotope values in offshore sites indicate that the available stock of nutrients far exceeds the demand. Sedimentation rates in the bay are not high and range from 5 to 54 cm/ka. This could be attributed to trapping efficiency by shoreline aquatic macrophyte. Furthermore, the mean accumulation rates of organic carbon and nitrogen for the bay, range from 6.92 to 57.25 gC/m2/yr and 0.51 to 4.37 gN/m2/yr, respectively, and show that Magu Bay is currently experiencing eutrophication. Tanzania Journal of Science Vol. 31 (1) 2005: pp. 23-3

Sediment characteristics and hydrodynamic setting of reef platform sediments of Kunduchi, north of Dar es Salaam harbour

Sediment grain size distribution, carbonate content and current velocities for the Kunduchi area are used to determine its hydrodynamic setting and sediment characteristics. The sediment mean grain size generally decreases northward. The sediments consist of medium to coarse sand south of the Tegeta River and fine sand further north. The sediment sorting values, which lack a north-south trend, vary from moderately well sorted to poorly sorted. The skewness of the sediment vary from near-symmetrical to very negatively skewed while the peakedness (kurtosis) varies from platykurtic to leptokurtic. The carbonate content increases with distance offshore, whereas the siliciclastic component decreases with distance offshore. The siliciclastic/carbonate transition of the investigated area is characterized by a narrow transition with no inter-fingering of the two facies. The lack of inter-fingering of the two facies suggests that there is a limited offshore transport of land-derived sediments. This is further supported by the drogue experiment results, which shows that the predominant surface current flows northward irrespective of the tidal phase Tanzania Journal of Science Vol. 32 (1) 2006: pp. 37-5

Sources and diagenetic alteration of organic matter in coastal waters at the vicinity of Dar es Salaam, Tanzania.

The stable isotope compositions of organic carbon (OC) and nitrogen, contents of OC and nitrogen, and C/N ratios for four short cores, which were recovered from Msansani Bay (WO 57), off Msimbazi River (WO 76) and Dar es Salaam harbour (WO 62 and WO 74), are used to document temporal variation in the sources and diagenetic alteration of organic matter (OM). The &#948;15N and &#948;13C values at sites WO 57, WO 62 and WO 74 show a trend of down-core decrease to the base of the core, and such a down-core trend could be related to diagenetic loss of compounds enriched in 13C and 15N. At site WO 76, the &#948;13C values show two zones that are depleted in 13C. These two zones correspond to black-greasy layers that were observed during sampling, and low &#948;13C values probably is an indication that the material was derived from the continent. Furthermore, these two zones have elevated values of OC, nitrogen and C/N ratios. The stable isotopes of OC and nitrogen show inter-sites differences with enrichment in 15N in the following order: WO 57 < WO 74 < WO 62 < WO 76. A similar inter-core difference is observable for the 13C, but the enrichment show a reversed order. The inter-site differences in 13C and 15N could be related to the sources of OM. In the Msansani Bay the material is derived from seagrasses while in the Dar es Salaam harbour and Msimbazi micro-bay, large proportion is derived from the continent. Other parameters of OC, nitrogen and C/N ratios for these three sites show a slight down-core decrease to the base of the core, probably the result of diagenetic alteration of OM. Highest contents of OC and nitrogen are observable at site WO 62 owning to preferential preservation of OM due to high input of more refactory material derived from the continent and lack of tidal flushing. Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 39-5

Hydrologic fragmentation-induced eutrophication in Dove Sound, Upper Florida Keys, USA

Anthropogenic impacts to island systems can have deleterious effects on coastal aquatic ecosystems. These effects can alter water quality, primary production as well as habitat. Land development often fragments hydrologic connectivity within aquatic ecosystems forcing alterations in nutrient transport and increases the potential for eutrophication. Dove Sound, a tidal lagoon located in the Upper Florida Keys on Key Largo, has been subjected to anthropogenic influences of land development during the last century. To investigate these influences a short sediment core was collected from within Dove Sound and investigated using 210Pb dating, stable isotopes of carbon and nitrogen, and sedimentary pigments. Results indicated that Dove Sound has undergone eutrophication and the primary producer community structure has shifted from dominantly macrophytic to a system that supports substantial algal production. While septic waste was a possible source for eutrophication, low δ15N did not support this conclusion. However, the timing of the shifts in Dove Sound along with indicators of anoxia leads to the conclusion that fragmentation caused by the construction of a railroad was the root cause. The hydrologic fragmentation reduced the flushing rates, thereby enhancing anoxic conditions in the system and increasing the internal nutrient loading

Changes in global nitrogen cycling during the Holocene epoch

Human activities have doubled the pre-industrial supply of reactive nitrogen on Earth, and future rates of increase are expected to accelerate. Yet little is known about the capacity of the biosphere to buffer increased nitrogen influx. Past changes in global ecosystems following deglaciation at the end of the Pleistocene epoch provide an opportunity to understand better how nitrogen cycling in the terrestrial biosphere responded to changes in carbon cycling. We analysed published records of stable nitrogen isotopic values (δ(15)N) in sediments from 86 lakes on six continents. Here we show that the value of sedimentary δ(15)N declined from 15,000 years before present to 7,056 ± 597 years before present, a period of increasing atmospheric carbon dioxide concentrations and terrestrial carbon accumulation. Comparison of the nitrogen isotope record with concomitant carbon accumulation on land and nitrous oxide in the atmosphere suggests millennia of declining nitrogen availability in terrestrial ecosystems during the Pleistocene-Holocene transition around 11,000 years before present. In contrast, we do not observe a consistent change in global sedimentary δ(15)N values during the past 500 years, despite the potential effects of changing temperature and nitrogen influx from anthropogenic sources. We propose that the lack of a single response may indicate that modern increases in atmospheric carbon dioxide and net carbon sequestration in the biosphere have the potential to offset recent increased supplies of reactive nitrogen in some ecosystems

Lapita diet in remote oceania: new stable isotope evidence from the 3000-year-old Teouma site, Efate Island, Vanuatu.

Remote Oceania was colonized ca. 3000 BP by populations associated with the Lapita Cultural Complex, marking a major event in the prehistoric settlement of the Pacific Islands. Although over 250 Lapita sites have been found throughout the Western Pacific, human remains associated with Lapita period sites are rare. The site of Teouma, on Efate Island, Vanuatu has yielded the largest burial assemblage (n=68 inhumations) of Lapita period humans ever discovered, providing a unique opportunity for assessing human adaptation to the environment in a colonizing population. Stable isotope ratios (δ13C, δ15N, δ34S) of human bone collagen from forty-nine Teouma adults were analyzed against a comprehensive dietary baseline to assess the paleodiet of some of Vanuatu's earliest inhabitants. The isotopic dietary baseline included both modern plants and animals (n=98) and prehistoric fauna from the site (n=71). The human stable isotope data showed that dietary protein at Teouma included a mixture of reef fish and inshore organisms and a variety of higher trophic marine (e.g. marine turtle) and terrestrial animals (e.g. domestic animals and fruit bats). The domestic pigs and chickens at Teouma primarily ate food from a C3 terrestrial environment but their δ15N values indicated that they were eating foods from higher trophic levels than those of plants, such as insects or human fecal matter, suggesting that animal husbandry at the site may have included free range methods. The dietary interpretations for the humans suggest that broad-spectrum foraging and the consumption of domestic animals were the most important methods for procuring dietary protein at the site. Males displayed significantly higher δ15N values compared with females, possibly suggesting dietary differences associated with labor specialization or socio-cultural practices relating to food distribution