Dinoflagellate cyst and pollen assemblages as tracers for marine productivity and river input in the northern Gulf of Mexico

Both marine dinoflagellate cysts and terrestrially derived pollen and spores are abundant in coastal sediments close to river mouths, making sediment records from such settings ideal to simultaneously study land– ocean climate interactions, marine productivity patterns and freshwater input over time. However, few studies consider the combined calibration of these palynological proxies in modern coastal sediments offshore from rivers, which is needed to strengthen the interpretation of paleoreconstructions. Here, we analyze the palynological content of marine surface sediments along land–sea transects off the Mississippi and Atchafalaya river mouths in the northern Gulf of Mexico (GoM) and test three palynological indices which are often employed in the paleo-domain: (i) the abundance of cysts of heterotrophic and autotrophic dinoflagellates (dinocysts) as a tracer for primary productivity (H /A ratio) and (ii) the ratio between non-bisaccate pollen and bisaccates (P /B) as well as (iii) the ratio between pollen (excluding bisaccates) and dinocysts (P /D), which are both tracers for river input and distance to the coast. Our results show that dinoflagellate cysts are most abundant on the shelf, where heterotrophic dinocyst species dominate coastal assemblages in reach of the river plume, while autotrophic taxa are more present in the oligotrophic open ocean. This is clearly reflected in decreasing H/A values further offshore. Individual dinocyst taxa also seem to inhabit specific niches along an onshore–offshore transect, linked to nutrient availability and proximity to the turbid river plume. The highest pollen concentrations are found close to the Mississippi river mouth and mostly represent a mixture of local coastal and upstream vegetation, whereas bisaccate pollen was most abundant further offshore of the Mississippi river. Multivariate redundancy analysis (RDA) performed on both pollen and dinocyst assemblages, a set of environmental parameters, and the three palynological ratios showed that net primary productivity was the most important variable influencing the dinocyst assemblages, likely as the result of nutrient input. Additionally, the RDA confirmed that the H/A ratio indeed seems to track primary productivity, while the P /B ratio results in a robust indicator for distance to the coast, and the P /D ratio better reflects river input. Together, our data confirm and further specify the suitability of these three palynological ratios in river-dominated coastal margins as proxies for (past) marine productivity and distance to the coast and river

Organic carbon stored in a thermokarst affected landscape on Baldwin Peninsula, Alaska

As Arctic warming continues and permafrost degrades, more organic carbon (OC) will be decomposed in high northern latitudes. Still, uncertainties remain in the quality and quantity of OC stored in permafrost. This study presents OC data from permafrost deposits on the Baldwin Peninsula, West-Alaska. We analyzed cryostratigraphical, biogeochemical and biomarker parameters of yedoma- and drained thermokarst lake basin (DTLB) deposits as well as thermokarst lake sediments to identify the size and quality of OC pools in ice-rich permafrost. Here we show that two thirds of soil OC in this region are stored in frozen DTLB deposits and that the lake sediments have the highest volumetric OC content. The n-alkane distribution shows, however, that OC stored in yedoma is of higher quality than that stored in DTLB deposits. These findings highlight the importance of molecular OC analysis for determining the potential future greenhouse gas emissions from thawing permafrost

Organic carbon characteristics in yedoma and thermokarst deposits on Baldwin Peninsula, West-Alaska

As Arctic warming continues and permafrost thaws, more soil and sedimentary organic carbon (OC) will be decomposed in northern high latitudes. Still, uncertainties remain in the quantity and quality of OC stored in different deposit types of permafrost landscapes. This study presents OC data from deep permafrost and lake deposits on the Baldwin Peninsula which is located in the southern portion of the continuous permafrost zone in West-Alaska. Sediment samples from yedoma and drained thermokarst lake basin (DTLB) deposits as well as thermokarst lake sediments were analyzed for cryostratigraphical and biogeochemical parameters and their lipid biomarker composition to identify the size and quality of belowground OC pools in ice-rich permafrost on Baldwin Peninsula. We provide the first detailed characterization of yedoma deposits on Baldwin Peninsula. We show that three quarters of soil organic carbon in the frozen deposits of the study region (total of 68 Mt) is stored in DTLB deposits (52 Mt) and one quarter in the frozen yedoma deposits (16 Mt). The lake sediments contain a relatively small OC pool (4 Mt), but have the highest volumetric OC content(93 kg m-3) compared to the DTLB (35 kg m-3) and yedoma deposits (8 kg m-3), largely due to differences in the ground ice content. The biomarker analysis indicates that the OC in both yedoma and DTLB deposits is mainly of terrestrial origin. Nevertheless, the relatively high carbon preference index of plant leaf waxes in combination with a lack of degradation trend with depth in the yedoma deposits indicates that OC stored in yedoma is less degraded than that stored in DTLB deposits. This suggests that OC in yedoma has a higher potential for decomposition upon thaw, despite the relatively small size of this pool. These findings highlight the importance of molecular OC analysis for determining the potential future greenhouse gas emissions from thawing permafrost, especially because this area close to the discontinuous permafrost boundary is projected to thaw substantially within the 21st century

From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kirkels, F. M. S. A., Ponton, C., Galy, V., West, A. J., Feakins, S. J., & Peterse, F. From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system. Journal of Geophysical Research-Biogeosciences, 125(1), (2020): e2019JG005270, doi:10.1029/2019JG005270.We investigate the implications of upstream processes and hydrological seasonality on the transfer of soil organic carbon (OC) from the Andes mountains to the Amazon lowlands by the Madre de Dios River (Peru), using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids. The brGDGT signal in Andean soils (0.5 to 3.5 km elevation) reflects air temperature, with a lapse rate of −6.0 °C/km elevation (r 2 = 0.89, p < 0.001) and −5.6 °C/km elevation (r 2 = 0.89, p < 0.001) for organic and mineral horizons, respectively. The same compounds are present in river suspended particulate matter (SPM) with a lapse rate of −4.1 °C/km elevation (r 2 = 0.82, p < 0.001) during the wet season, where the offset in intercept between the temperature lapse rates for soils and SPM indicates upstream sourcing of brGDGTs. The lapse rate for SPM appears insensitive to an increasing relative contribution of 6‐methyl isomer brGDGTs produced within the river. River depth profiles show that brGDGTs are well mixed in the river and are not affected by hydrodynamic sorting. The brGDGTs accumulate relative to OC downstream, likely due to the transition of particulate OC to the dissolved phase and input of weathered soils toward the lowlands. The temperature‐altitude correlation of brGDGTs in Madre de Dios SPM contrasts with the Lower Amazon River, where the initial soil signature is altered by changes in seasonal in‐river production and variable provenance of brGDGTs. Our study indicates that brGDGTs in the Madre de Dios River system are initially soil derived and highlights their use to study OC sourcing in mountainous river systems.The brGDGT analyses were supported by NWO‐Veni grant 863.13.016 to F.P. This material is based upon work supported by the US National Science Foundation under grant EAR‐1227192 to A. J. W. and S. J. F. for the river fieldwork and lipid purification. In Perú, we thank the Servicio Nacional de Áreas Naturales Protegidas por el Estado (SERNANP) and personnel of Manu and Tambopata National Parks for logistical assistance and permission to work in the protected areas. We thank the Explorers' Inn and the Pontifical Catholic University of Perú (PUCP), as well as the Amazon Conservation Association for the use of the Tambopata and Wayqecha Research Stations, respectively. For river fieldwork assistance, we thank M. Torres, A. Robles, and A. Cachuana. Soil samples were contributed by Andrew Nottingham and Patrick Meir. Logistical support was provided by Y. Malhi, J. Huaman, W. Huaraca Huasco, and other collaborators as part of the Andes Biodiversity and Ecosystems Research Group ABERG (www.andesresearch.org). We thank Dominika Kasjaniuk for technical support at Utrecht. Two anonymous reviewers have provided valuable comments that have helped to improve this manuscript. Geochemical and brGDGT data are available in the PANGAEA Data Repository (Kirkels et al., 2019) and can be accessed at https://doi.pangaea.de/10.1594/PANGAEA.90617

Decreased Westerly moisture transport leads to abrupt vegetation change in northern Central Asia during late MIS3: Evidence from Zeketai Loess, Ili Basin

Moisture transportation to (semi-)arid Central Asia is influenced by the interplay of multiple atmospheric circulation patterns. The mechanism underlying the hydroclimate evolution in CA has recently received increasing attention. In northeastern CA, most of the precipitation falls in summer, counter to other westerly-dominated regions of CA where precipitation maximizes in boreal winter. The marine isotope stage (MIS) 3 offers a unique opportunity to examine the mechanisms driving hydroclimate variations during warm periods of the ice age in the CA. However, due in part to reliable proxies for reconstruction, the hydroclimate and vegetation change during MIS3 in northeastern CA have not been well studied. Here, we use grain size and magnetic susceptibility data from the Zeketai loess profile to reconstruct historical shifts in precipitation patterns during MIS3 in the Ili Basin from arid Central Asia (ACA). Notably, abrupt changes around 35 ka are evident in the grain size and magnetic susceptibility records, reflecting a transition to reduced wind intensity and increased aridity. Subsequent analysis of plant leaf waxes (n-alkanes), hydrogen isotopic composition of n-alkanes (δDn-alkane), and stable carbon isotope ratios of total organic carbon (δ13Corg) within the loess profile indicates a concurrent shift towards more shrubs, aligning with reduced moisture availability. The δ13Corg record further elucidates that during MIS3, the Ili Basin predominantly harbored C3 vegetation. However, the alteration in δ13Corg and δDn-alkane to more positive values at approximately 35 ka substantiates the transition to more drought-resistant vegetation. Furthermore, we link the paleo-hydroclimate variations to the decrease in solar radiation and the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) and Siberian High during late MIS3. This joint influence weakens the Westerly winds and subsequently diminishes the conveyance of moisture to CA

The dispersal of fluvially discharged and marine, shelf-produced particulate organic matter in the northern Gulf of Mexico

Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge since the commonly used bulk organic matter (OM) parameters do not reach the required level of source- and process-specific information. Here, we analyzed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi-Atchafalaya River (MAR) system, as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil-microbial organic matter (SMOM), fluvial, higher plant, and marine-produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of SMOM and fluvial OM near the Mississippi River (MR) mouth (BIT Combining double low line 0.6, FC321,15 > 50 %), which rapidly decrease further away from the river mouth (BIT < 0.1, FC321,15 < 20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore and reveal that marine producers (diatoms, dinoflagellates, coccolithophores) have different spatial distributions, indicating their preferred niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean suggest that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our combined lipid biomarker and palynology approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental margin

Isotopic and magnetic proxies are good indicators of millennial-scale variability of the East Asian monsoon

Past East Asian Monsoon variability has been reconstructed using oxygen isotopes of cave speleothems as well as proxy indicators from Chinese loess sequences. However, where the speleothem record is dominated by precession cycles, loess magnetic susceptibility primarily shows a glacial-interglacial pattern. Here we generate a ~ 130,000 years high resolution record of plant wax hydrogen isotopes from a loess section on the western Chinese Loess Plateau that can directly be compared with both speleothem oxygen isotopes and with magnetic susceptibility from the same section. We find that variations in our plant wax hydrogen isotope record follow the precessional pattern of the speleothem record as opposed to the glacial-interglacial changes in magnetic susceptibility. We propose that hydrogen isotopes mainly record precipitation during the growing season, whereas magnetic susceptibility represents an annual climate signal, including precipitation and temperature. Our findings imply that summer vs annual climate variability is driven by distinct orbital forcings

Recovery from multi‐millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

Enhanced nutrient input and warming have led to the development of low oxygen (hypoxia) in coastal waters globally. For many coastal areas, insight into redox conditions prior to human impact is lacking. Here, we reconstructed bottom water redox conditions and sea surface temperatures (SSTs) for the coastal Stockholm Archipelago over the past 3000 yr. Elevated sedimentary concentrations of molybdenum indicate (seasonal) hypoxia between 1000b.c.e.and 1500c.e. Biomarker-based (TEX86) SST reconstructions indicate that the recovery from hypoxia after 1500c.e.coincided with a period of significant cooling (similar to 2 degrees C), while human activity in the study area, deduced from trends in sedimentary lead and existing paleobotanical and archeological records, had significantly increased. A strong increase in sedimentary lead and zinc, related to more intense human activity in the 18(th)and 19(th)century, and the onset of modern warming precede the return of hypoxia in the Stockholm Archipelago. We conclude that climatic cooling played an important role in the recovery from natural hypoxia after 1500c.e., but that eutrophication and warming, related to modern human activity, led to the return of hypoxia in the 20(th)century. Our findings imply that ongoing global warming may exacerbate hypoxia in the coastal zone of the Baltic Sea