Metallic nanoparticle enrichment at low temperature, shallow CO₂ seeps in Southern Italy

Abstract

We report on metal enrichment along a natural pH gradient owing to increased CO₂ degassing at cold, shallow seeps of Vulcano Island in the Mediterranean Sea, off Sicily. We assessed composition of unfiltered and filtered seawater (< 100 nm) along acidic zones ranging between ambient and pH 5, and showed that most seep derived elements are present as nanoclusters which then aggregate into larger colloids while mixing with ambient seawater along a pH gradient. Size and elemental composition of such naturally occurring nanoparticles assessed by modern characterisation methods were in good agreement with the results from conventional analytical methods.

We provide analytical evidence for the presence in the water column of a large fraction of seep derived elements (e.g. approximately 50% of iron, over 80% of Mn, 100% of Cr, S and Zn) in the form of nano sized particles (e.g. < 100 nm) even at typical open ocean pHs. We launch in situ sampling protocols and sample preparation procedures for multi-method suitable to obtain accurate measurements on nanoparticles from environmental samples. Based on our results a first insight to the formation of natural nanoparticles at cold CO₂ seeps is presented and the persistence of such nano-clusters in the surrounding seawater is stipulated.

Introduction

Deep sea hydrothermal vents are considered “chemical reactors” for the formation of both organic and inorganic compounds and their transformation through abiotic and biotic processes (Luther, 2004), and are a significant source of metals to the ocean. Steep pH gradients combined with metal enriched fluid emissions typical at these high temperature deep sea hydrothermal vents (Kadar et al., 2005) are conditions known to facilitate the formation of Fe-rich nanoparticles (NPs) (Kennedy et al., 2004, Kim et al., 2008, Yucel et al., 2011, Wu et al., 2011). Recent research suggests that an unknown fraction of this flux escapes from precipitation/flocculation reactions partly due to stabilisation by organic ligands (Sander and Koschinsky, 2011, Toner et al., 2009, Bennett et al., 2008), and due to nanoparticles reported to be kinetically stable (Yucel et al., 2011).

Contrasting such high temperature dominated system emissions from the shallow, high temperature vents (cold seeps) have metal concentration several orders of magnitude lower (Kim et al., 2008). Formation of metallic nanoclusters may therefore be less likely to occur in cold seeps, but cannot be ruled out and has not been adequately investigated to date. Nanoparticle formation under the reducing conditions typical at cold seeps is a result of mobilisation of metals due to CO₂ seepage in the sediment owing to dissolution of particles and release of adsorbed metals on the surface of the solids, and also due to changes of decomposition rates of biogenic particulate material due to decreased pH and pE (Ardelan et al., 2009). The significance of the contribution of such metallic nanoclusters to the biogeochemical cycling of Fe at shallow underwater CO₂ emissions is unknown and needs to be addressed.

Whilst sampling challenges in extreme environments (Kadar and Powell, 2006) and the lack of analytical techniques for detection and characterisation of natural NPs (Lead et al., 2000) have prevented in depth study of the actual morphology, mineralogy and chemical properties of the NPs emitted in deep sea hydrothermal fluids until recently, shallow seeps may be a more accessible site for such studies. The contrasting geological settings, distributions, biogeochemistry, and dynamics of vent and seep ecosystems render distinctive characteristics ranging from the source of reduced compounds available for microbial transformation into energy through chemical oxidation, to the nature of the substratum. It is fair to assume therefore that the environmental fate of NPs within the shallow, cold, sunlit settings of CO₂ seeps will be quite different from deep-sea vents, which warrants our comparative discussion.

The shallow fumaroles within the geothermal field in Southern Italy have recently been in the spotlight (Spencer-Hall et al., 2008) providing natural laboratory conditions to study physiological adaptations to ocean acidification i.e. valuable predictors of future ocean ecology owing to their elevated CO₂ flux. The geothermal field from the Levante Bay comprises both terrestrial and submarine gas emissions emerging a few metres from the beach at about 1 metre depth. The resulting gas emissions from the hydrothermal aquifer are associated with magmatic fluids coming from the crater fumaroles and modified by low-temperature subsurface processes (Chiodini et al., 1995). The gas emissions are characterised by high CO₂ contents > 90% volume and a significant variability of H₂S ranging from 0.8 to 2.5% volume (Capaccioni et al., 2001). The large variability of H₂S in the gas discharge can be attributed to the interaction between metal sulphides and hydrothermally altered volcanic and weakly acid waters. Thermal water discharges from the Istmo‐Porto di Levante beach further complicate the water chemistry of the bay (Aiuppa et al., 2000) by their input of elevated conductivity, high Na and Cl content, very low pH and Eh and high content of volatile metals, due to acid gas condensation (Aiuppa et al., 2000). Such underwater volcanic emissions create a natural pH gradient that provide excellent sites to study the environmental behaviour (i.e. stability, mobility, and persistence) of metal-rich and organic NPs from various volcanic fluids and/or gas. Taking advantage of the geochemically distinct, shallow fumaroles as natural sites of NP emission it is uniquely possible to study transport and fate of NPs under natural environmental conditions, investigate their tendency to aggregate and interact with other particles under different bio-geo-chemical settings. Understanding their environmental fate and behaviour may help us distinguish such naturally present NPs from engineered NPs, a fundamental nanoscience challenge.

In an effort to better understand environmental fate and behaviour of natural NP we report on the prevalent types of metal-rich natural NPs along a natural pH gradients created by submarine volcanic gas emissions at a geochemically distinct geothermal system in Vulcano Island in South Italy. Our aim is to conduct an integrated study of these hydrothermal NPs to reveal their composition, size distribution, surface topology and chemical reactivity. Specific aims were to characterise these NPs along a pH gradient determined by distinct CO₂ fluxes and investigate the relationships between NP structure and the geochemical settings at hydrothermal seeps. In addition, we provide key information on methods suitable for detection of naturally present NPs.