Is Oxygenation Related to the Decomposition of Organic Matter in Cryoconite Holes?

Cryoconite is a sediment occurring on glacier surfaces worldwide which reduces ice albedo and concentrates glacier surface meltwater into small reservoirs called cryoconite holes. It consists of mineral and biogenic matter, including active microorganisms. This study presents an experimental insight into the influence of sediment oxygenation on the cryoconite ability to produce and decomposition of organic matter. Samples were collected from five glaciers in the Arctic and the European mainland. Cryoconite from three glaciers was incubated in stagnant and mechanically mixed conditions to imitate inter-hole water–sediment mixing by meltwater occurring on glaciers in Northern Hemisphere, and its effect on oxygen profiles and organic matter content. Moreover, we investigated short-term changes of oxygen conditions in cryoconite from four glaciers in illuminated and dark conditions. An anaerobic zone was present or approaching zero oxygen in all illuminated cryoconite samples, varying in depth depending on the origin of cryoconite: from 1500 µm from Steindalsbreen (Scandinavian Peninsula) and Forni Glacier (The Alps) to 3100 µm from Russell Glacier and Longyearbreen (Arctic) after incubation. Organic matter content varied between glaciers from 6.11% on Longyearbreen to 16.36% on Russell Glacier. The mixed sediment from the Forni Glacier had less organic matter than stagnant, the sediment from Longyearbreen followed this trend, but the difference was not statistically significant, while the sediment from Ebenferner did not differ between groups. Our results have implications for the understanding of biogeochemical processes on glacier surfaces, the adaptation of organisms to changing physical conditions due to abrupt sediment mixing, but also on the estimation of productivity of supraglacial systems.

Pyrite-lined shells as indicators of limited oxygen exposure time and inefficient bioirrigation in the Holocene-Anthropocene stratigraphic record

Although the depth of bioturbation can be estimated on the basis of ichnofabric, the time scale of sediment mixing and irrigation by burrowers that affects carbonate preservation and biogeochemical cycles is difficult to estimate in the stratigraphic record. However, pyrite linings on interior of shells can be a signature of slow mixing and irrigation rate because they indicate that shells of molluscs initially inhabiting oxic sediment zones were immediately and permanently sequestered in reduced microenvironments where molluscan biomass and associated microbial coatings stimulated sulfate reduction and pyrite precipitation. A high abundance of pyrite-lined shells in the stratigraphic record can thus be diagnostic of limited net exposure of labile tissues to O2 even when the seafloor is inhabited by abundant burrowing infauna as in the present-day northern Adriatic Sea. Here, we reconstruct this sequestration pathway (1) by assessing preservation and postmortem ages of pyrite-lined shells of the hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) by evaluating whether an independently-documented decline in bioturbation, driven by eutrophication and seasonal hypoxia during the 20th century, affected the frequency of pyrite-lined shells in the stratigraphic record of the northern Adriatic Sea. First, at prodelta sites with high sedimentation rate, linings of pyrite framboids form rapidly in near-surface sediment zones as they appear already in interiors of shells and in intra-shell conchiolin layers younger than 10 years and occur preferentially in well-preserved and articulated shells with periostracum and relatively high concentrations of amino acids. Second, increments deposited in the early 20th century contain < 20 % of shells with pyrite at the Po prodelta and 30–40 % at the Isonzo prodelta, whereas the late 20th century increments possess 50–80 % of shells with pyrite at both locations. At sites with slow sedimentation rate, the frequency of pyrite linings is low (< 10–20 %). Third, the upcore increase in the frequency of pyrite-lined shells positively correlates with an abrupt increase in maximum shell size and biomass of V. gibba. Therefore, the upcore increase in the frequency of pyrite-lined shells indicates that sediment mixing and bioirrigation rates declined during the 20th century, leading to higher sequestration of pyrite-lined shells during the late 20th century. We hypothesize that the permanent preservation of pyrite linings within the shells of V. gibba in the subsurface stratigraphic record was allowed by slow recovery of infaunal communities frequently interrupted by seasonal hypoxic events, leading to the dominance of surficial sediment modifiers with low irrigation potential. Abundance of well-preserved shells lined by pyrite exceeding ~10 % per assemblage in apparently well-mixed sediments in the deep-time stratigraphic record can be an indicator of short net exposure of shells to O2 and inefficient bioirrigation. Fine-grained prodelta sediments in the northern Adriatic Sea deposited since the mid-20th century, with high preservation potential of reduced microniches, can represent taphonomic and early-diagenetic analogues of deep-time skeletal assemblages with pyrite linings.

Sediment Deposition within Rainwater: Case Study Comparison of Four Different Sites in Ikorodu, Nigeria

Building roofs represents a critical pathway for sediment mixing with rainwater. This study aims to explore the correlation between roof-top deposited sediment matter in the different areas of the Ikorodu Local Government Area in Lagos, Nigeria. The deposition rate on the roof was studied for 34 weeks in total (i.e., 17 weekly analyses in the rainy season and 17 weekly analyses in the dry season). The total deposition was collected by a 10 inch funnel and directed into a 5 L container, which was partially filled with sterilised water. The roof-top deposition in four different areas was inspected and analysed. The four areas were selected based on the levels of sanitation and vegetation. The experimental results showed that the enumerated total depositions in different areas were higher in the dry season than the rainy season, with the highest deposition occurring in the Harmattan period. The data obtained from this study have evidenced that the contamination from roof-harvested rainwater can mainly be attributed to atmospheric deposition. Another key factor was the hygiene and sanitation of the harvesting areas, including the gutter, pipes and proximity to animal faeces.

Bioturbation of Thalassinoides from the Lower Cambrian Zhushadong Formation of Dengfeng area, Henan Province, North China

Bioturbation plays a critical role in sediment mixing and biogeochemical cycling between sediment and seawater. An abundance of bioturbation structures, dominated by Thalassinoides, occurs in carbonate rocks of the Cambrian Series 2 Zhushadong Formation in the Dengfeng area of western Henan Province, North China. Determination of elemental geochemistry can help to establish the influence of burrowing activities on sediment biogeochemical cycling, especially on changes in oxygen concentration and nutrient regeneration. Results show that there is a dramatic difference in the bioturbation intensity between the bioturbated limestone and laminated dolostone of the Zhushadong Formation in terms of productivity proxies (Baex, Cu, Ni, Sr/Ca) and redox proxies (V/Cr, V/Sc, Ni/Co). These changes may be related to the presence of Thalassinoides bioturbators, which alter the particle size and permeability of sediments, while also increase the oxygen concentration and capacity for nutrient regeneration. Comparison with modern studies shows that the sediment mixing and reworking induced by Thalassinoides bioturbators significantly changed the primary physical and chemical characteristics of the Cambrian sediment, triggering the substrate revolution and promoting biogeochemical cycling between sediment and seawater.

Assessing behavioural traits of benthic foraminifera: implications for sediment mixing

The assessment of behavioural traits of marine organisms is increasingly recognized as a key issue to understanding their role in ecosystem processes such as bioturbation and nutrient cycling. The movement ability of intertidal foraminifera suggest that they may have a role, yet to be quantified, in benthic-pelagic coupling through their movement on the sediment surface, at the sediment-water interface and within the sediment. In this context, we investigated the behavioural traits of 5 benthic foraminiferal species typical of European temperate mudflats under standardized trophic light and temperature conditions. Behavioural traits related to motion of Ammonia tepida, Haynesina germanica, Cribroelphidium williamsoni, Miliammina fusca and Quinqueloculina seminula were assessed through their travelled distance, velocity, tortuosity of the path, position in the sediment and activity index. By analogy with macrofauna bioturbation functional groups, we describe the studied foraminifera as biodiffusor species with 3 sub-groups defined according to their vertical position in the sediment. C. williamsoni belongs to the epifaunal-biodiffusors, A. tepida and H. germanica belong to the surficial-biodiffusors, and Q. seminula and M. fusca are considered gallery-biodiffusors. Our results further suggest that features such as velocity, activity and tortuosity may mediate sediment-mixing intensity. Therefore, Q. seminula, H. germanica and C. williamsoni, which are the most active species, would have a larger effect on particle reworking rates than the less active A. tepida and M. fusca. Our results suggest that benthic foraminifera may play an underestimated role in bioturbation processes.