14C constraints to the Holocene aggradation of three major tributary valleys of the Tiber River in Rome

By means of core data collected in a dedicatedly performed 35 m deep borehole (CAF-S2), seven 14C ages constraints to sediment aggradation, and new chronostratigraphic information from previously unpublished and new borehole stratigraphic logs, we reconstruct the post-glacial aggradation of the alluvial successions occurring in three tributary valleys draining the left hydrographic basin of the Tiber River in central Rome: Caffarella, Murcia and Grottaperfetta valleys. We combine the new information with that achieved in the major Tiber Valley by previous work providing evidence for a widespread marked increase in the water transport capacity within the entire catchment, highlighted by the sudden deposition of gravel with diameter of the pebbles ≤2 cm, occurred 5200 through 4000 yr BP. In the Tiber Valley, this gravel horizon is emplaced at the bottom of a fining-upwards aggradational succession that, 4000 through 2800 yr BP, re-filled a deep paleo-incision excavated in the former 5500 yr BP alluvial plain, suggesting that a lowering of the base level is the cause of the increase in capacity of transport within the Tiber catchment. In contrast, a corresponding phase of non-deposition is observed in the lowest stretches of the investigated tributary valleys, concomitant with the accumulation of a 4 m thick fine gravel horizon in the higher portion of the Caffarella Valley. Such framework is interpreted as the result of retrograde erosion from the Tiber mouth towards the inland catchments, which didn't affect the tributary valleys due to the short lasting of the sea-level fluctuation.
While no evidence for a significant sea-level fall 5200 through 4000 yr BP occurs in the global records, this time span is broadly coinciding with the "4.2 ka Event" which is considered a global cooling and drying period lasting 4.3 through 3.8 ka. Therefore, we propose that the observed "Mid-Holocene transgressive phase" affecting the Tiber catchment 5.2 - 3.8 ka represents the local evidence of the 4.2 ka Event, possibly extending back in time its beginning, and we suggest that further investigation is necessary in order to verify whether any significant sea-level fluctuation is associated to this climate event.

通过专门执行的35米深钻孔（CAF-S2）收集的核心数据，结合七个14C年龄对沉积物加积的限制以及来自先前未发表和新钻孔地层柱状图的全新年代地层信息，我们重建了罗马中部蒂伯河左岸水系三条支流谷地（Caffarella、Murcia和Grottaperfetta谷地）中冲积层在冰后期加积的过程。我们将新信息与先前在主要蒂伯河谷地取得的研究成果相结合，提供了在整个流域范围内水运能力显著增加的证据，这一增加特征为直径≤2厘米的砾石在5200至4000年前突然沉积。在蒂伯河谷地，这一砾石层位于向上变细的加积层底部，从4000至2800年前，重新填充了5500年前前冲积平原中的深古河道，这表明基面下降是蒂伯河流域运移能力增加的原因。相比之下，在研究支流谷地的下游，观察到相应的非沉积阶段，与Caffarella谷地较高部位积累的4米厚细砾石层同时出现。这一结构被解释为从蒂伯河河口向内陆流域逆向侵蚀的结果，由于海平面波动持续时间较短，因此并未影响支流谷地。尽管5200至4000年前全球记录中没有发现显著的海平面下降证据，但这一时间段与被认为是全球冷却和干燥期的“4.2ka事件”大致吻合，该事件持续时间为4.3至3.8ka。因此，我们提出，影响蒂伯河流域的“中全新世海侵阶段”（5.2-3.8ka）代表了4.2ka事件的局部证据，并可能将其开始时间向前延伸，我们建议进一步调查以验证是否任何显著的海平面波动与这一气候事件相关。