Permeable marine sediments: Overturning an old paradigm

沙质沉积物覆盖约70%的大陆架和大多数海滩，是重要的海岸带生态系统之一。生态系统的运转需要物质和能量的传递，微生物驱动的关键元素生物地球化学循环是其中的关键，而有机质是地球系统元素生地化循环的重要驱动力。沙质沉积物由于缺乏有机物和其他活性物质，过去通常被认为是“生物地球化学沙漠”和“没有生命的港湾”，因此过去的生地化循环研究往往忽略了沙质沉积物这个分布广泛的生态系统。然而，值得注意地是，越来越多的观测证据正在推翻这种被误导的观点，沙质沉积物中存在着快速和活跃的碳氮生地化循环过程。

Sandy sediments, which cover about 70% of continental shelves and most beaches, are one of the important coastal zone ecosystems. Ecosystems require the transfer of matter and energy to function, of which microbially driven biogeochemical cycles of key elements are key, and organic matter is an important driver of the Earth system’s elemental biogeochemical cycles. Sandy sediments are often thought of as geochemical deserts that harbor no life because they are usually poor in organic matter and other reactive substances. Based on the belief that significant reactions and fluxes and a dynamic ecology require large standing stocks of reactants and organic matter, sandy sediments are neglected. However, new observations are changing this misguided view that rapid and active carbon and nitrogen biogeochemical cycling processes are occurring in sandy sediments.

图 两种主要的渗透陆架沉积物形式:硅酸盐砂(上图)和碳酸盐砂(下图)（来自Huettel et al.(2003)）

Fig. The ranges of dominant transport mechanisms in marine sediments and the two main forms of permeable shelf sediments: silicate sands (upper inset) and carbonate sands (lower inset). Adapted from Huettel et al. (2003).

首先，沙质沉积物中存在快速的水渗透及其携带的外部有机质和营养成分进入。沙质沉积物具有对水流的高渗透性特征，在潮汐、波浪和洋流等外力驱动下造成沙质沉积物中的压力波动，驱动上覆水与沙质沉积物的交换，导致孔隙水在沉积物中的分散混合，其发生速度可能是分子扩散的数倍。在上覆水与孔隙水的快速交换下，沙质沉积物充当于过滤系统，可以将上覆水中携带的氧气、悬浮颗粒、细菌、溶解有机质和营养盐吸附在孔隙中，研究表明，浅海大陆架在底流的作用下沙质沉积物的溶质通量可提高10倍。这部分进入沙质沉积物的外源物质改变了原有的氧化还原环境，并驱动了各生地化过程的发生。

First, there is rapid water infiltration in sandy sediments as well as the entry of external organic matter and nutrients carried by them. Sandy sediments are characterized by high permeability to water flow, and external forces such as tides, waves and currents cause pressure fluctuations in sandy sediments that drive the exchange of overlying water with sandy sediments, resulting in the dispersed mixing of pore water in the sediments, which can occur at a rate several times faster than molecular diffusion. With the rapid exchange of overlying water and pore water, the sandy sediment acts as a filtering system that can adsorb oxygen, suspended particles, bacteria, dissolved organic matter and nutrients carried by the overlying water in the pores. Studies have shown that the solute flux of sandy sediments on shallow continental shelves can be increased by a factor of 10 under the action of bottom currents. The exogenous material entering sandy sediments alters the original redox environment and drives the occurrence of various biotechnological processes.

具体来说，氧气的供应改变了原有沉积物中的氧化还原环境，促进内部有机颗粒的降解，为碳氮生地化循环过程提供底物。比如沙质沉积物对浮游生物降解的能力随着沉积物渗透性的增加而增加。此外，上覆水与孔隙水的交换与沉积物内部的生地化反应的相互作用可能造成渗透层内复杂的三维地球化学空间结构，水体的平流会增强内部的浓度梯度，而扩散过程则起到消散和均匀化的作用。

Specifically, the supply of oxygen alters the redox environment in the original sediment, promotes the degradation of internal organic particles, and provides substrates for the carbon and nitrogen biogeochemical cycling processes. For example, the ability of sandy sediments to degrade plankton increases with increasing sediment permeability. Furthermore, the interactions between these reactions, the inflow of overlying water, and the outflow of porewater can lead to complex three-dimensional geochemical zonations within permeable beds. Advection thus creates and enhances concentration gradients, in contrast to diffusion, which acts to dissipate and homogenize.

第二，近岸沙质沉积物中发现存在大量的底栖生物和微生物，是沙质沉积物物质循环过程的重要参与者。沙滩由于沙质海岸动态的、物理上不稳定的环境，往往缺乏大型动物物种，平均每个海滩有1到40个大型物种，全世界总共有100到1000个。然而，在每个海滩上那些鲜为人知的小型生物（小型底栖生物，meiofauna）能达到100多种，主要包括线虫、羽形虫和涡虫等，全世界有1000到10000种。此外，在沙滩中微藻以及各类微生物——细菌、真菌、纤毛虫等，同样也是种类丰富的。在潮汐的作用下，富氧水周期性冲刷的开放性孔隙网络结构和随之而来的动态化学作用支持了近岸沙滩内的物种多样性，而这些微生物和微型动物在沙质沉积物中有机物矿化，各营养物质循环过程中扮演重要角色。

Second, the presence of a large number of benthic organisms and microorganisms found in nearshore sandy sediments is an important participant in the material cycling process of sandy sediments. Due to the dynamic and physically unstable environment of the sandy littoral, the beach consequently is species-poor in macro-fauna, with generally one to 40 species per beach, and a total of 100 to 1,000 worldwide. However, poorly-known smaller organisms known as meiofauna may exceed 100 species per beach; these are mainly nematodes, harpacticoids, and turbellarians, and species number 1,000 to 10,000 worldwide. Likewise, micro-algae and microbes-bacteria, fungi, ciliates, etc.—are also abundant. The high diversity of the meio- and micro-organisms in the lower beach is supported by the large network of open pores regularly flushed by oxygen-rich water and an ensuing dynamic chemistry, and these microorganisms and microfauna play an important role in the mineralization of organic matter in sandy sediments and in the cycling of various nutrients.

第三，颗粒物有机质含量与其粒径大小呈负相关关系，颗粒越大，有机质含量越少，粒径约10-20 μm的颗粒有机碳含量为1-5%，当粒径大于200 μm时有机碳含量低于0.05%。由于沙质沉积物通常由大粒径的石英砂或碳酸盐砂颗粒组成，通常具有较低的有机质含量，因此过往研究认为沙滩不存在活跃的生物地球化学过程。但现有研究发现，颗粒有机质的可降解性却与其粒径成正相关关系，大颗粒有机质含量虽然较低，但其可降解性却高于小颗粒有机质。比如，粒度小于100 μm时，降解常数为0.01-0.5 y-1，而粒度为大于 200μm时，降解常数为0.05-3.5 y-1。这种与传统认知相反的现象表明，沙质沉积物可能是地球化学循环的重要场所，其重要性远远超过以往的设想。

Third, the organic matter content of particulate matter is negatively correlated with its particle size. Maximum organic carbon contents (1-5%) are at about 10-20 um median grain size from which they rapidly decrease to < 0.05% at grain sizes > 200 um. Since sandy sediments are usually composed of large-size quartz sand or carbonate sand particles, which usually have a low organic matter content, previous studies have suggested that there are no active biogeochemical processes at the beach. However, existing studies have found that the degradability of particulate organic matter is positively correlated with its particle size, with larger particles having a lower organic matter content but higher degradability. For example, values increase from 0.01-0.5 y-1 at grain sizes < 100μm to 0.05-3.5 y-1 at grain sizes > 200μm. This phenomenon, contrary to conventional knowledge, suggests that sandy sediments may be an important site for geochemical cycling that is far more important than previously thought.

第四，沙质沉积物的有机质输入不仅来自于上覆水体的生产和截留，底栖生物的初级生产是沙质沉积物有机质的重要来源之一。在浅水的边缘海区域，由于水体浊度较低，光渗透深度较大， 0.1%的表层光可到达海底，而这部分光可支持底栖环境中藻类和利用光的细菌的生长。研究发现大约30%的大陆架海床（总面积约为3.4 x 10 8 km2）会发生显著的底栖生物初级生产过程。

Fourth, organic matter input to sandy sediments does not only come from production and sequestration in the overlying water column, and primary production by benthic organisms is one of the important sources. In shallow marginal marine regions, where light penetration depths are large due to low water turbidity, 0.1% of surface light can reach the seafloor which can support the growth of algae and light-using bacteria in the benthic environment. Significant benthic primary production processes are found to occur in about 30% of the shelf seabed (total area of about 3.4 x 10 8 km2).

有限的实测数据表明沙质沉积物底栖初级生产速率的范围从接近零到大于800 mg C m-2 d-1，平均值为230 mg C m-2 d-1。基于平均值估计全球的底栖生物初级生产速率为2.9 x 10 14 g C y-1 (0.3 Gt y-1)。而C. duarte得出海草(大部分生长在砂质沉积物上)的生产速率为0.6 Gt y-1。沙质沉积物的底栖生产与海草生产相当，说明沙质沉积物中底栖生物的生产是重要的有机质来源。虽然这些估算存在很大不确定性，但它们的大小表明了底栖生物的生产对沙质沉积物的生物地球化学循环的具有潜在重要性。

The limited measured data indicate benthic primary production rates in sandy sediments range from near zero to >800 mg C m-2 d-1, with a mean value of 230 mg C m-2 d-1. The global rate of benthic primary production is estimated to be 2.9 x 10 14 g C y-1 (0.3 Gt y-1) based on the mean value. In contrast, C. duarte yields a production rate of 0.6 Gt y-1 for seagrasses (most of which grow on sandy sediments). The benthic production in sandy sediments is comparable to seagrass production, suggesting that benthic production in sandy sediments is an important source of organic matter. Although there are large uncertainties in these estimates, their magnitudes indicate the potential importance of benthic production to the biogeochemical cycling of sandy sediments.

足够的证据表明沙质沉积物在地球系统生地化循环过程中具有重要的地位，越来越多的研究开始关注沙质沉积物中的元素生地化循环，探索其中的碳氮过程机制、环境调控机理及其生态与气候效应是未来的重要科学问题。

There is sufficient evidence that sandy sediments are important in the biogeochemical cycling of the Earth system, and more and more studies are focusing on the biogeochemical cycling of elements in sandy sediments. Exploring the mechanisms of carbon and nitrogen processes, environmental regulation mechanisms and their ecological and climatic effects is an important scientific orientation in the future.

图 高潮(a)和低潮(b)期间沙质沉积物海底地下水排放及可能的相关氮循环过程示意图(改编自 Santos et al. (2021))。

Fig. Schematic diagram of submarine groundwater discharge from sandy sediments during high tide (a) and low tide (b) and possible associated nitrogen cycling processes. Adapted from Santos et al. (2021).

参考文献：

Reference:

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/EO082i011p00133-01

                                                 投稿者：符春蔚

                                                 审核导师：谭萼辉