The age and ability of salt marshes to accumulate and sequester carbon is often assessed using the carbon isotopic signatures (Δ¹⁴C and δ¹³C) of sedimentary organic matter. However, transfers of allochthonous refractory carbon (C_RF) from the watershed to marshes would not represent new C sequestration. To better understand how refractory carbon (C_RF) inputs affect assessments of marsh age and C sequestration, Δ¹⁴C and δ¹³C of both total organic carbon (TOC), C_RF, and non-C_RF organic matter fractions were measured in salt marshes from four contrasting systems on the North Atlantic coast. To our knowledge, no salt marsh sediment study has considered refractory or allochthonous carbon in carbon budget calculations or the impact on chronologies. Stable and radiogenic isotope data suggest that while TOC was dominated by autochthonous plant inputs, C_RF was dominated by locally recycled or allochthonous C, the delivery of which was controlled by the size and slope of each watershed. Steep-gradient rivers analyzed delivered Δ¹⁴C-depleted C_RF to their estuarine marshes, while the site located in the low-gradient river was associated with larger C_RF content. Finally, the marsh isolated from riverine input contained the least fraction of TOC as C_RF. Laterally transported C_RF caused only a small offset in Δ¹⁴C in relation to TOC in low-gradient systems (average Δ¹⁴C offset was −44.4 and −24.2‰ at each location). However, the presence of allochthonous Δ¹⁴C-depleted C_RF in sediments of steep-gradient rivers led to large overestimates of the time of organic matter deposition (i.e. apparent age was older than the ‘true’ time of deposition) (Δ¹⁴C offset ranged from −170.6 to −528.9‰). Further, reliance on TOC or loss on ignition analyses to calculate C sequestration by marshes might produce overestimates of at least as much as 10 to 20% since neither account for the lateral transport of allochthonous carbon.

年龄和盐沼积累和固碳的能力是使用碳同位素特征（Δ经常评估¹⁴ C和δ ¹³ C）的沉积的有机物质。但是，将异质耐火碳（C _RF）从分水岭转移至沼泽不会代表新的C固存。为了更好地了解如何碳耐火材料（C _RF）输入影响沼泽年龄和C封存，Δ的评估¹⁴ C和δ ¹³二者有机碳总量（TOC），C的Ç _RF，和非C _RF在北大西洋沿岸四个对比系统的盐沼中测量了有机物含量。据我们所知，没有盐沼沉积物研究在碳预算计算或对时间的影响中考虑了耐火或异质碳。稳定的放射成因同位素数据表明，尽管TOC受到本地植物输入的支配，但C _RF受本地回收或异源C支配，其输送受每个流域的大小和坡度控制。陡梯度进行分析河流递送Δ ¹⁴ C-耗尽Ç _RF其河口沼泽，而位于在低梯度河流站点用较大的C相关联_的RF内容。最后，与河流输入隔离的沼泽地的TOC含量最少，为C _RF。横向运Ç _RF只引起小的Δ偏移¹⁴相对于TOC在低梯度系统C（平均Δ ¹⁴ C的失调是-44.4和-24.2‰在每个位置处）。然而，外来Δ的存在¹⁴ C-耗尽Ç _RF中导致的有机物沉积的时间大高估陡梯度河流沉积物（即表观年龄比沉积的“真实”时间以上）（Δ ¹⁴C偏移范围为-170.6至-528.9‰）。此外，依靠TOC或燃烧分析的损失来计算沼泽的C固存可能会产生至少10％到20％的高估，因为这两种方法均未考虑异源碳的横向传输。