From land to the ocean: the interplay between allochthonous and autochthonous contribution to particles in nepheloid layers of the Cariaco Basin, Venezuela

TitleFrom land to the ocean: the interplay between allochthonous and autochthonous contribution to particles in nepheloid layers of the Cariaco Basin, Venezuela
Publication TypeJournal Article
Year of Publication2019
AuthorsLorenzoni L, Thunell RC, Benitez-Nelson CR, Montes E, Varela R, Astor YM, Muller-Karger F
JournalJGR Biogeosciences
Date Published09/2019
Abstract

Abstract

Lithogenic sediment input to the Cariaco Basin on the eastern Venezuelan shelf is controlled by small mountainous rivers (SMR). The Cariaco Basin is also an area of high phytoplankton productivity as a result of strong Trade Wind‐driven coastal upwelling. Characterizing the sources that supply particulate organic carbon (POC) to the deep Cariaco Basin is important for interpreting the paleoclimate record stored in its sediments. We measured suspended POC in the four main rivers draining into the Caraiaco Basin: the Tuy, Unare, Neveri and Manzanares, between September 2008 and September 2009, and conducted basin‐wide oceanographic cruises in September 2008 (rainy season) and March 2009 (upwelling season). Riverine concentrations of dissolved and particulate organic carbon (DOC and POC, respectively) in the four rivers were comparable to observations made in similar tropical SMR systems (POC was between 0.3‐2 mg C l‐1; DOC was between 100‐300 μM). Within the basin, the geochemical composition of surface particles and bottom nepheloid layers (BNL) changed with season. During the rainy season, the isotopic composition of both surface particles and BNL was characteristic of continentally‐derived material (δ13Corg, ~‐30 ‐ ‐26‰), while during upwelling, the composition shifted to values more typical of marine sources (δ13Corg, ~‐24 ‐ ‐20‰). SMRs represent an important component of the global carbon budget, which are often overlooked in ocean carbon budgets and also in paleoclimate studies of coastal environments.

Plain Language Summary

We generally tend not to take into consideration small rivers when we are thinking about the transport of suspended and particulate matter from land to the sea at a global level. However, there are a lot of small rivers, and those that start in mountains are of particular importance because they have the potential of transporting a lot of material very fast to the ocean. When this material enters a location in the ocean that is low in oxygen and deep, it can be preserved for long periods of time, and tell a story of what conditions on land were like at the time it was transported. Such a location is the Cariaco Basin, on the continental shelf of Venezuela, in northern South America. In the Cariaco Basin, small local rivers seasonally discharge carbon and sediment which are effectively funneled to the deepest portions of the basin via turbid, underwater layers of sediment; the influence of this land‐derived material changes the composition of the turbid layers seasonally. Because Cariaco is devoid of any oxygen, this material is preserved, undisturbed, for centuries. This research helped us better understand what these small, local rivers are discharging into the Cariaco Basin, which ultimately help us better appreciate the importance of small rivers (they are small, but there are many!), and what gets stored in the deep sediments of the Cariaco Basin, thereby enabling us to reconstruct a more accurate story of change over the northern South American continent.

DOI10.1029/2019JG005288
Refereed DesignationRefereed

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