Transformation Of Sulfur Species And Its Effects On The Trace Heavy Metals Bioavailability In Antarctic Lake Sediments

Sulfur is one of the nutrient elements,its biogeochemical migration,transformation and cycling couple to the decomposition of organic matters,the recycling of carbon,nitrogen and phosphorus,water acidification and control of metal availability through formation of insoluble metal sulfides.These couplings play a vital role in the function of aquatic and wetland ecosystems.Penguins can transport amounts of nutrients?C,N,S and P?,trace heavy metals and persistent organic pollutants in the form of guanos from ocean to lacustrine systems and formed the ornithogenic sediments,which has been used successfully to infer the changes of past penguin populations and climate and environment.The biogeochemical transformations of such nutrients,especially sulfur,in those ornithogenic sediments,however,are rarely studied.In this study,various sulfur species,physicochemical indexes and trace heavy metal fractions in an ornithogenic sediment?C1?from Ardley Island and a pristine lake sediment?C2?from Fildes Peninsula,Antarctic,were analyzed to study the transformation of sulfur species and its restriction on the bioavailability of trace heavy metals.We obtain the following results and conclusions.?1?The total organic carbon,total phosphorus,total nitrogen and total sulfur in C1 are more fluctuant and much higher than those in C2,indicating they are mainly from the inputs of penguin guanos.The relatively consistent trends indicate they are mainly from the inputs of penguin guanos.For the sulfur species in C1 sediments,the organic sulfur?Org-S?is the main form,which ranges from 220.49?g/g to 10299.75?g/g with a mean of 2675.49?g/g.It appears a low value at 8 cm,then increases,with a high value at 15 cm and a maximum value at 52 cm.Sulfate(SO₄^2-)is the main form of inorganic sulfur,ranges from 191.49?g/g to 6981.99?g/g with a mean of 986.20?g/g.Its vertical distribution trend is opposite to Org-s at the top 30 cm.It reaches a higher value at 8 cm,then decreases,with a lower value at 15 cm,and then fluctuates at lower values.It rises sharply after 45cm and reaches a maximum value at the bottom.The acid volatile sulfur?AVS?is a production of dissimilatory sulfate reduction,ranging from 3.16?g/g to 1550.68?g/g with a mean of 331.11?g/g.The content of AVS is relatively high in the sub-surface layer?8-19 cm?,with a minimum value at 45 cm and a maximum value at the bottom;While the pyrite sulfur?CRS?ranges from 7.22?g/g to 312.43?g/g,with a mean of 61.95?g/g.The minimum value appears at 8 cm,with a high value at 15 cm,then it fluctuates at a lower level.It rises sharply after 45 cm and reaches a maximum value at the bottom.In C2 sediments,the vertical distribution of total organic carbon?TOC?,total phosphorus?TP?,total nitrogen?TN?and total sulfur?TS?shows a trend of fluctuation,and the Org-S and SO₄^2-are the main sulfur species;CRS is the main form of reduced inorganic sulfur in C2 sediments in contrast to the AVS in C1,suggestes that AVS has been transformed into CRS effectively.Our study suggestes that high level of organic matters in C1likely limite the transformation of AVS to CRS.?2?The content of Cu and Cd in C1 sediment is 56.32-778.76?g/g?mean of298.76?g/g?and 0.65-9.99?g/g?mean of 4.54?g/g?,respectively,and the oxidizable fraction is their main chemical form.The content of Zn is 116.53-881.30?g/g?mean of 391.49?g/g?,with the main forms of weak acid extraction and residual fraction.In contrast,Cu ranges from 32.46 to 49.50?g/g?mean of 39.35?g/g?in C2 sediments,with a main form of the weak acid extraction.The content of Zn was 44.03-62.98?g/g?mean of 51.58?g/g?,with the main form of oxidizable fraction.Cd ranges from0.19 to 0.27?g/g?mean of 0.24?g/g?,with the main forms of weak acid extraction and residual fraction.The vertical distribution trend of trace heavy metals in C1sediments is similar to the physicochemical indices,indicating they are both from the inputs of penguin guanos.The vertical distribution of trace heavy metals in C2sediments shows a fluctuation trend,which is speculated to be related to environmental changes in the area.The ratio of SEMs/AVS in sediments was used to determine the metals bioavailability and toxicity.The?SEM/AVS ratio in C1sediments is less than that in C2,indicates that the mobility of trace heavy metals in C1 was lower than that of C2.The lower ratio of?SEM/AVS appears in subsurface and deeper reduction zone in C1 and in deeper reduction zone in C2,which is probably related to the vertical distribution of the sulfate reducing bacteria.In penguin droppings,the exchangeable fraction of Cd occupy more than 50%of its total quantity;In C1 sediments,however,Cd was existed mainly in the forms of organic matter and sulfide bounded,indicated that although a large amount of toxic and harmful trace heavy metals were transported to the freshwater environment,the high nutrients brought by penguins stimulated strong sulfate reduction in the sediments,and restricted the biological availability of trace heavy metals.