『Abstract
We studied the changes in major elements and organic carbon concentrations
during the initial stage of the mixing of the black (Rio Negro)
and the White (Rio Solimoes) waters in the Amazon River basin
to understand the geochemical processes that could control the
redistribution between particulate and dissolved fractions. Water
samples were collected at six stations including the Rio Negro
and the Rio Solimoes and four stations downstream
from the confluence. The relative contributions of the two tributaries
were determined using a triple tracer approach (δ18O,
δD, Cl-). Particulate (＞0.2μm) and dissolved (＜0.2μm)
concentrations of major elements (Ca, Mg, Fe, Al, Si) and organic
carbon (POC and DOC) were measured. Major elements in the particulate
fraction were found to have a nonconservative behaviour in the
initial stage of the mixing due to mineral removal. In the dissolved
fraction, only the DOC, Fe, and Mn behaved nonconservatively.
The Fe losses could be due to preferential removal of Fe bound
to N-rich organic matter (OM) and/or to preferential removal of
Fe oxyhydroxides. The increasing dissolved manganese content in
the dissolved phase is explained by a reductive dissolution of
manganese oxides due to massive inputs of phenolic-rich OM from
the Rio Negro. The amount of DOC removed from the water column
in the initial stage of the mixing would represent 4％ of the total
annual DOC flux of the Amazon River at the reference gauging station
of Obidos.
Keｙwords: Amazon; Mixing zone; Isotopic tracer; Organic matter;
Iron; Manganese; Sorption』
『我々は粒子と溶存分間の再分配をコントロールしうる地球化学的過程を理解するために、アマゾン川流域の黒水（Rio Negro）と白水（Rio Solimoes）の混合の最初の段階の間の主要元素と有機炭素濃度の変化を研究した。水試料は、Rio NegroとRio Solimoesを含む6地点および合流点から下流の4地点で採取された。2つの支流の相対的な寄与の程度は、3種のトレーサー法（δ18O、δD、Cl-）を用いて決定された。主要元素（Ca、Mg、Fe、Al、Si）と有機炭素（POCとDOC）の粒子（＞0.2μm）と溶存（＜0.2μm）濃度が測定された。粒子分の主要元素は、鉱物が除かれるため混合の初期段階では保存されない振る舞いをすることがわかった。溶存分では、DOC、FeおよびMnのみが保存されない挙動をした。Feの損失は、Nに富む有機物（OM）に結合したFeの選択的な除去および／またはFeオキシ水酸化物の選択的除去によるだろう。溶存相中の溶存マンガン含有量が増加するのは、Rio Negroからの多量のフェノール系有機物のインプットによりマンガン酸化物が還元されて溶解するということで説明される。混合の最初の段階で水柱から除去されるDOCの量は、Obidosの基準計測地点でのアマゾン川の合計年間DOCフラックスの4％に相当するだろう。』
1. Introduction
2. Materials and methods
2.1. Sampling
2.2. Filtration
2.3. Unfiltered waters and dissolved load
2.4. Suspended load
3. Results
3.1. δD-δ18O and Cl- tracing of the
mixing process
3.2. Distribution among the dissolved and particulate fractions
3.3. Elemental budget in the mixing zone
3.3.1. Ca, Mg, K and Na
3.3.2. Organic carbon
3.3.3. Aluminium, iron, manganese
4. Discussion
4.1. Particulate major elements removal during the initial
mixing
4.2. Behaviour of organic matter in the initial mixing zone
4.3. Behaviour of Fe and Al in the early stage mixing
4.4. Manganese input during the initial mixing
5. Conclusion
Acknowledgements
References