Sediment organic carbon

Source: ICRAM

Introduction

Carbon in marine sediments occurs as organic carbon (OC) intimately linked to the metabolic processes and remains of plants and animals and as carbon contained within biogenic and abiogenic carbonate minerals. Successful determination of sediment organic carbon relies upon the separation of organic from inorganic carbon (IC). Several methods of analysis have been employed for the separation of organic from inorganic forms of carbon (reviewed in  Verardo et al.,1990). Although the determination of total carbon (TC) content in marine sediments is straightforward by a carbon-hydrogen-nitrogen (CHN) elemental analyzer, that of organic carbon presents difficulties. This is because organic material is a complex mixture, and some of these components may be lost from the sediment at the temperatures both above and below those for the loss of other materials, such as structural water and carbonates. The determination of OC is usually a lengthy procedure, involving weighing of the sediment sample, careful pretreatment of the sample by a suitable acid to remove IC, drying of the sediment, reweighing of sediment and analysis by CHN analyzer (Tung & Tanner, 2003).

Required

Falcon test-tube 

Broad knife

Ruler (to measure sediment height)

Centrifuge (to extract interstitial water from the sediments)

Freeze-dryer (to lyophilize the sediments)

Mortar (to homogenize  sediments)

Sieve with 200 µm mesh

Micro-analytical balance   

Procedure

Sampling:

·        Collect the sediment sample, by means of a corer device;

·        Remove the upper 1 cm layer of the sediment, homogenize in a clean vessel, transfer to a centrifuge tube and store on ice during transit to the laboratory

Sample processing:

·        Samples  are centrifuged to remove interstitial water and lyophilized (freeze-dried)

·        Freeze-dried samples are gently pulverized using clean mortar and pestle and subsequently sieved at 200 µm;

·        6-10 mg of homogenized sediment are transferred to a special soft Silver cup (for elemental analysis)

·        Treat each sample by adding 20 µl of 1N HCl, to oxidize inorganic carbonate. If effervescence continues, this step of the acidification procedure must be repeated;

·        Wait for 10 minutes;

·        Dry sample in 60°C oven  for 10 minutes,

·        Addition of 20 µl of HCl  1:1. If effervescence continues, this step of the acidification procedure must be repeated;

·        Wait for 10 minutes;

·        Dry sample in 60°C oven  for 10 minutes

·        Addition of 20 µl of HCl 25%. If effervescence continues, this step of the acidification procedure must be repeated;

·        Wait for some minutes;

·        Desiccate samples in oven, drying overnight at 60°C