ALBERT

Description: The relative abundance of the C32 1,15 long-chain alkyl diol (LCD) is an emerging proxy for the input of riverine aquatic particulate organic carbon (POC) into coastal oceans. This compound has the potential to complement other established proxies reflecting riverine terrestrial POC input and allows for a more nuanced assessment of riverine POC export to coastal seas. The current understanding of this proxy is, however, limited. In this study, we compare different indices for riverine sediment input to coastal marine waters (i.e. C32 1,15-LCD, BIT index and Fe/Ca ratio) in a source-to-sink assessment in the Amazon River drainage system and the northeast South American continental margin, and we test their down-core applicability in a marine gravity core containing late Pleistocene fluvial Amazonian sediments. We show that the relative abundance of the C32 1,15-LCD is highest in water bodies with low flow velocity and low turbidity such as the downstream portion of lowland tributaries and floodplain lakes. Relative C32 1,15-LCD abundance is lowest in Andean white water tributaries where autotrophic productivity is hindered by high turbidity and high flow velocity. We also find that suspended particulate matter from all major tributaries during the extreme 2015 dry season has a similar LCD distribution to that of floodplain lakes. This indicates that the chemical composition of the tributaries is less relevant for the LCD distribution than their physical properties such as flow velocity and turbidity. Results from marine surface sediments offshore the Amazon River estuary show significant positive correlations between all three studied proxies. In contrast, we find that the relative C32 1,15-LCD abundance in the down-core record is anti-correlated to the BIT index and Fe/Ca ratio. While BIT index and Fe/Ca ratio show high (low) values during Heinrich stadials (Dansgaard-Oeschger interstadials), the C32 1,15-LCD proxy shows the opposite signal. BIT values are also higher during Marine Isotope Stage (MIS) 2 than during MIS 3, in contrast to trends in the C32 1,15-LCD proxy. We posit that this pattern arises from a reduction in relative C32 1,15-LCD abundance and total LCD productivity in the Amazon River during MIS 2 when less-humid conditions and lower sea level led to reduced area of floodplains. During Heinrich stadials, Andean sediment input increased and led to higher turbidity that resulted in lower C32 1,15-LCD production. Our study shows that major changes in water discharge, sediment transport and river morphology can lead to discrepancies between the BIT index and the relative abundance of the C32 1,15-LCD. Thus, we suggest that Amazonian aquatic and terrestrial POC pools had contrasting responses to changes related to both climate (e.g. increased Andean precipitation) and river morphology (e.g. steeper along-channel slope due to falling and low stand sea level).

Description: Paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin, transport and deposition of biomarkers.Here, we contribute to the understanding of these processes by analyzing long-chain n-alkanes from the Amazon River system. We use the dD composition of long-chain n-alkanes from river bed sediments from the Amazon River and its major tributaries, as well as marine core-top samples collected off northeastern South America as tracers for different source areas. The d13C composition of the same compounds is used to differentiate between long-chain n-alkanes from modern forest vegetation and petrogenic organic matter. Our d13C results show depleted d13C values (-33 to -36‰) in most samples, indicating a modern forest source for most of the samples. Enriched values (-31 to -33‰) are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source. Long-chain n-alkane dD analyses show more depleted values for the western tributaries, the Madeira and Solimões Rivers (-152 to -168‰), while n-alkanes from the lowland tributaries, the Negro, Xingu and Tocantins Rivers (-142 to -154‰), yield more enriched values. The n-alkane dD values thus reflect the mean annual isotopic composition of precipitation, which is most deuterium-depleted in the western Amazon Basin and more enriched in the eastern sector of the basin. Samples from the Amazon estuary show a mixed long-chain n-alkane dD signal from both eastern lowland and western tributaries. Marine core-top samples underlying the Amazon freshwater plume yield dD values similar to those from the Amazon estuary, while core-top samples from outside the plume showed more enriched values. Although the variability in the river bed data precludes quantitative assessment of relative contributions, our results indicate that long-chain n-alkanes from the Amazon estuary and plume represent an integrated signal of different regions of the onshore basin. Our results also imply that n-alkanes are not extensively remineralized during transport and that the signal at the Amazon estuary and plume includes refractory compounds derived from the western sector of the Basin. These findings will aid in the interpretation of plant wax-based records of marine sediment cores collected from the adjacent ocean.

Description: The Amazon River transports large amounts of terrestrial organic carbon (OCterr/ from the Andean and Amazon neotropical forests to the Atlantic Ocean. In order to compare the biogeochemical characteristics of OCterr in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments, we analysed riverbed sediments from the Amazon mainstream and its main tributaries as well as marine surface sediments from the Amazon shelf and fan for total organic carbon (TOC) content, organic carbon isotopic composition (δ13CTOC/, and lignin phenol compositions. TOC and lignin content exhibit positive correlations with Al = Si ratios (indicative of the sediment grain size) implying that the grain size of sediment discharged by the Amazon River plays an important role in the preservation of TOC and leads to preferential preservation of lignin phenols in fine particles. Depleted δ13CTOC values (-26.1 to -29.9 ‰) in the main tributaries consistently correspond with the dominance of C3 vegetation. Ratios of syringyl to vanillyl (S = V) and cinnamyl to vanillyl (C = V) lignin phenols suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Although the Amazon basin hosts a rich diversity of vascular plant types, distinct regional lignin compositions are not observed. In the marine sediments, the distribution of δ13CTOC and 38 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100 mg OC) values implies that OCterr discharged by the Amazon River is transported north-westward by the North Brazil Current and mostly deposited on the inner shelf. The lignin compositions in offshore sediments under the influence of the Amazon plume are consistent with the riverbed samples suggesting that processing of OCterr during offshore transport does not change the encoded source information. Therefore, the lignin compositions preserved in these offshore sediments can reliably reflect the vegetation in the Amazon River catchment. In sediments from the Amazon fan, low lignin content, relatively depleted δ13CTOC values and high (Ad = Al)V ratios indicating highly degraded lignin imply that a significant fraction of the deposited OCterr is derived from petrogenic (sourced from ancient rocks) sources.

Description: Bulk organic parameters and stable and radiocarbon isotope compositions of organic carbon (δ13COC and Δ14COC) as well as various biomarkers (lignin phenols, plant waxes etc.) have been used to investigate the biogeochemical characteristics of organic carbon in the Amazon River system. However, the source, concentration and distribution pattern of lignin on the Amazon shelf and fan has not been assessed so far. In particular, the compound-specific stable carbon isotope compositions (δ13C) of lignin phenols have not been characterized in the Amazon River system. In order to study the distribution of lignin in the lower Amazon basin and its dispersal on the shelf and fan, we used riverbed sediments from the Amazon mainstream and its main tributaries and marine surface sediments on the Amazon shelf and fan. The samples were analyzed for particulate organic carbon content (POC), δ13COC, lignin phenol compositions and compound-specific δ13C of individual lignin phenols. The concentrations of aluminium and silicon (Al/Si) were used as a proxy for grain size[1]. The POC content in the main tributaries ranged from 0.13 to 3.99 wt-% and increased with Al/Si ratio in each tributary. δ13COC varied from -26.1‰ to -29.9‰ VPDB in riverbed sediments. Lignin content (represented by Λ8, sum of eight lignin phenols in OC, expressed as mg/100mg OC) ranged from 0.73 to 6.91 and is positively related with Al/Si ratio in the main tributaries except for the Xingu River, in which Λ8 decreased with Al/Si. Ratios of syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V) varied from 0.70 to 1.51 and 0.08 to 0.47, respectively, suggesting that the dominant source of lignin is non-woody angiosperm tissue. The ratios of vanillic acid to vanillin (Ad/Al)v (0.26-0.71) and syringic acid to syringaldehyde (Ad/Al)s (0.15-0.57) indicated relatively fresh, non-degraded lignin. In marine sediments, the δ13COC ranged from -18.6‰ to -26.7‰ and is correlated with the Λ8 value (0.04-2.01). The decreasing Λ8 value along the coast from the Amazon River mouth towards the northwest implies that lignin is distributed by the North Brazil Current. A main plant source of non-woody angiosperm tissue was indicated by the S/V (0.59-1.62) and C/V (0.10-0.43) ratios on the marine samples. The agreement between riverbed and marine sediments suggests that processing of POC during transport from the basin to offshore does not change the plant source information of lignin. Highly degraded lignin on the Amazon fan and the southeast shelf is indicated by (Ad/Al)v (0.49-0.99). δ13C of lignin phenols of 9 marine sediments ranged from -28.6‰ to -33.3‰ and were consistently lower than δ13COC (-19.7‰ to -26.7‰). Depleted δ13C of lignin phenols indicate that the POC produced by the terrestrial biosphere is mainly derived from higher plants using C3 photosynthesis. References Bouchez, J., Galy, V., Hilton, R.G., Gaillardet, J., Moreira-Turcq, P., Pérez, M.A., France-Lanord, C., Maurice, L., 2014. Source, transport and fluxes of Amazon River particulate organic carbon: Insights from river sediment depth-profiles. Geochemica et Cosmochimica Acta 133, 280-298

Description: Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014): 3-13, doi:10.1016/j.palaeo.2014.05.030.

Description: Reconstructions of surface paleoceanographic conditions of the western equatorial Atlantic and past climates of the adjacent Northeast Brazilian (the "Nordeste") continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0° 20’ N on the equatorial Brazilian continental slope. The record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near- modern to approximately 110 Ka. High-resolution XRF analysis provides insight into the paleoclimate history of the Nordeste during the last glacial interval. Several large-amplitude and abrupt peaks are observed in the time series of Ti/Ca and are usually accompanied by peaks of Fe/K. Together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from Nordeste rivers into the Atlantic. Within the limits of dating accuracy, most Ti/Ca peaks correlate with Heinrich events in the North Atlantic. This record thus corroborates, and extends back in time, the previous record of Arz et al (1998) determined on sediment cores from farther southeast along the Nordeste margin. Stable oxygen isotopic analysis and Mg/Ca paleothermometry on the near- surface-dwelling planktonic foraminiferal species Globierinoides ruber find that mean sea-surface temperature (SST) during glacial time (20 to 55 Ka, n = 97) was 23.89 ± 0.79 °C and the mean SST during the late Holocene (0 to 5 Ka, n = 14) was 26.89 ± 0.33 °C. SSTs were 0.5 to 2 °C higher and inferred sea-surface salinities were lower during most of the periods of elevated Ti/Ca, thus, as observed in previous studies, the western equatorial Atlantic was warm (at least locally) and the adjacent southern tropical continent was wet at the same time that the high-latitude North Atlantic was cold. Using the SYNTRACE-CCSM3 fully coupled climate model with transient forcing for the period 22 Ka to present, we find that decreased transport of the North Brazil Current co-occurs with reduced Atlantic meridional overturning circulation, and colder-than-normal SSTs in the North Atlantic region. These simulated conditions are invariably associated with significantly increased precipitation in the Nordeste region.

Description: Funding for the cruise and post-cruise science was provided to PAB by NSF-OCE-0823650.