Spatial heterogeneity of sources of branched tetraethers in shelf systems: The geochemistry of tetraethers in the Berau River delta (Kalimantan, Indonesia)

The bulk organic matter composition (total organic carbon (TOC) content and d13CTOC) and composition of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGT) in surface sediments from 43 stations in the Berau River delta (east Kalimantan, Indonesia), including two coast-shelf transects and stations within the river mouth, were examined to reveal the spatial heterogeneity in these parameters in order to assess the impact of a tropical river loaded with suspended matter on the sedimentary organic matter in the shelf system. The high-resolution study showed that, despite the extensive transport of eroded soil material by the river to the sea, terrestrial organic matter and brGDGTs are only deposited on a relatively small part of the shelf. The concentrations of brGDGTs are highest (up to 120 µg g-1 TOC) in sediments deposited in and close to the mouth of the Berau River and their distribution indicates that they represent a mixture of soil-derived and river in-situ produced brGDGTs. Crenarchaeol concentrations reach 700 µg g-1 TOC in sediments deposited on the outer shelf due to Thaumarchaeotal production in shelf waters. This results in a strong gradient (0.93–0.03) in the BIT index, with high values in the river mouth and low values on the shelf. The decline in the BIT index is caused by both decreasing concentrations of the brGDGTs and increasing concentrations of crenarchaeol. The BIT index shows a highly significant but non-linear relationship with d13CTOC. On the shelf, in the area not under the direct influence of the Berau River, cyclic brGDGTs become relatively dominant, most probably due to in-situ production in the alkaline pore waters of the surface sediments. The spatial heterogeneity of sources of brGDGTs on the Berau shelf complicates the use of brGDGTs as temperature proxies. Application of the global soil calibration to sedimentary mixtures of brGDGTs in the river-influenced area of the shelf results in a severe underestimation of mean annual air temperature (MAT) by 6 °C. This is due to the mixed origin of the brGDGTs, which are not only derived from soil erosion but, likely, also from riverine production, as has been observed for other river systems.Comparison of the Berau shelf other shelf systems indicates that in-situ production of brGDGTs in shelf sediments is a widespread phenomenon that is especially pronounced at water depths of ca. 50–300 m. It is hypothesized that this is so because benthic in-situ production of heterotrophic brGDGT-producing bacteria is fueled by the higher delivery of fresh organic matter to these sediments as the consequence of higher primary productivity in shelf waters and a decreased mineralization due to the relatively short settling times of particles on the shelf. For palaeoclimatic studies of marine shelf sediments the application of brGDGTs as proxies is severely complicated by the heterogeneity of sources of brGDGTs. Comparison of the brGDGT composition of soils with those of shelf sediments may assist in deciding if sedimentary brGDGTs are predominantly derived from soil erosion. Several methods to do so are discussed

Depth-related distribution of a key gene of the tetraether lipid biosynthetic pathway in marine Thaumarchaeota

The distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGT) lipids synthesized by Thaumarchaeota has been shown to be temperature-dependent in world oceans. Depth-related differences in the ammonia monooxygenase (amoA) of Thaumarchaeota have led to the classification of ‘shallow’ and ‘deep water’ clusters, potentially affecting GDGT distributions. Here, we investigate if this classification is also reflected in a key gene of the thaumarchaeotal lipid biosynthetic pathway coding for geranylgeranylglyceryl phosphate (GGGP) synthase. We investigated metagenomic databases, suspended particulate matter and surface sediment of the Arabian Sea oxygen minimum zone. These revealed significant differences in amoA and GGGP synthase between ‘shallow’ and ‘deep water’ Thaumarchaeota. Intriguingly, amoA and GGGP synthase sequences of benthic Thaumarchaeota clustered with the ‘shallow water’ rather than with ‘deep water’ Thaumarchaeota. This suggests that pressure and temperature are unlikely factors that drive the differentiation, and suggests an important role of ammonia concentration that is higher in benthic and ‘shallow water’ niches. Analysis of the relative abundance of GDGTs in the Arabian Sea and in globally distributed surface sediments showed differences in GDGT distributions from subsurface to deep waters that may be explained by differences in the GGGP synthase, suggesting a genetic control on GDGT distributions

Sustainable Habitat Restoration: Fish, Farms, and Ecosystem Services

Biomass burning impacts biogeochemical cycling, vegetation dynamics and climate. However, interactions between fire, climate and vegetation are not well understood and therefore studies have attempted to reconstruct fire and vegetation history under different climatic conditions using sedimentary archives. Here we focus on levoglucosan, a thermal by-product of cellulose generated during biomass burning, and, therefore, a potential fire biomarker in the marine sedimentary archive. However, before levoglucosan can be applied as a biomass burning proxy in marine sediments, there is a need for studies on how levoglucosan is transported to the marine environment, how it is reflecting biomass burning on continents, as well as the fate of levoglucosan in the marine water column and during deposition in marine sediments. Here we present analyses of levoglucosan, using an improved Ultra High Pressure Liquid Chromatography-Electro Spray Ionization/High Resolution Mass Spectrometry (UHPLC-ESI/HRMS) method, in atmospheric particles, in particulate matter settling through the water column and in marine surface sediments on a longitudinal transect crossing the tropical North Atlantic Ocean at 12°N. Levoglucosan was detected in the atmosphere, although in low concentration, possibly due to the sampled particle size, the source area of the aerosols, or the short time interval of sampling by which large burning events may have been missed. In sinking particles in the tropical North Atlantic Ocean we find that levoglucosan deposition is influenced by a mineral ballast effect associated with marine biogenic particles, and that levoglucosan is not transported in association with mineral dust particles. Highest levoglucosan concentrations and seasonal differences in sinking particles were found close to continents and low concentrations and seasonal differences were found in the open ocean. Close to Africa, levoglucosan concentration is higher during winter, reflecting seasonal burning in northwestern Africa. However, close to South America levoglucosan concentrations appear to be affected by riverine transport from the Amazon River. In surface sediments close to South America, levoglucosan concentration is higher than in the middle of the Atlantic Ocean, implying that here the influence from the South American continent is important and perennial. Our study provides evidence that degradation of levoglucosan during settling in the marine water column is not substantial, but is substantial at the sediment–water interface. Nevertheless, levoglucosan was detected in all surface sediments throughout the tropical North Atlantic, indicating its presence in the marine sedimentary record, which reveals the potential for levoglucosan as a biomass burning proxy in marine sediments

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO2 and paleoproductivity [Discussion paper]

A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane, a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7 ‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6 ‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion : pCO2 and paleo productivity

A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane), a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle