Marine Sediments Remotely Unveil Long-Term Climatic Variability Over Northern Italy

Abstract

A deep understanding of natural decadal variability is pivotal to discuss recently observed climate trends. Paleoclimate proxies allow reconstructing natural variations before the instrumental period. Typically, regional-scale reconstructions depend on factors like dating, multi-proxy weighting and calibration, which may lead to non-robust reconstructions. Riverine records inherently integrate information about regional climate variability, partly overcoming the above mentioned limitation. The Po River provides major freshwater input to Eastern Mediterranean, as its catchment encompasses a large part of Northern Italy. Here, using historical discharge data and oceanographic measurements, we show that Po River discharge undergo robust decadal fluctuations that reach the Ionian Sea, ~1,000 km South of Po River delta, through propagating salinity anomalies. Based on this propagation, we use a high-resolution foraminiferal δ(18)O record from a sediment core in the Ionian Sea to reconstruct North Italian hydrological variability on millennial-scale for the first time. The reconstruction reveals highly significant decadal variability that persists over the last 2,000 years. Many reconstructed extremes correspond to documented catastrophic events. Our study provides the first millennial-scale reconstruction of the strength of decadal hydrological variability over Northern Italy. It paves the way to assess the persistence of large-scale circulation fingerprints on the North Italian climate.

Figure 1. Amplitude of salinity decadal variability along the Adriatic coast.

(a) Map of the Adriatic Sea with surrounding regions (made with the Interactive Data Language (IDL) software, IDL Version 8.3 EULA, Exelis Visual Information Solutions, Inc., Boulder, Colorado, USA), showing the location of the Po River and the path of the freshwater outflow along the Italian coastline, the dominant path of the inflowing currents along the Croatian coasts, and a schematic circulation pattern in the Gulf of Taranto. (b) Bathymetric map showing the Gallipoli Terrace in the Gulf of Taranto, Ionian Sea, and the location of the sediment cores studied by our group (see points on the map). The isotopic ratio δ¹⁸O was measured in core GT90/3 (red point on the map), drilled at (39° 45’ 53” N, 17° 53’ 33” E); the core was extracted at a depth of 178 m and has a length of 3.57 m. (c) Amplitudes of decadal variability of upper-layer salinity measured since the early 1940s in selected locations along the Eastern Italian coast (ranked from North to South).

Figure 2. Decadal fluctuations in Po River discharges, salinity profiles from the Western Adriatic and Northern Ionian seas, and δ¹⁸O series from the GT90/3 Ionian Sea core.

Panel a: Annual-average Po River discharges and associated decadal oscillations estimated by SSA (blue curve) and CWT (red curve), as described in the Supplementary information. Panel b: Average precipitation over the Po valley (relative deviations). Panel c: Upper-layer (0–20 m depth) salinity measured along the Western Adriatic coast. Panel d: Salinity at Gallipoli (at 0–20 and 0–150 m depths), Rimini (0–20 m depth) and in the Northern Ionian Sea (South of Calabria, 37° 40’ N, 16° 20’E, 0–150 m depth). Panel e: δ¹⁸O values (red points) measured in G. ruber of the core GT90/3 extracted from the Gallipoli terrace (Ionian sea) and δ¹⁸O of the calcite calculated using the Shackleton equation with salinity and temperature values at different depths corresponding to the same Gallipoli site (plotted anomalies are with respect to the average value of the 0–20 m layer).

Figure 3. Extrapolation of the Po River discharge before 1917 to the last 2 millennia (red curve).

As a reference for the last centuries, the Po discharge decadal component is superimposed (blue curve). Known major floods in the Po plain from 200 BC are represented by the histogram and the major ones by black triangles. The same weight is attributed to each flooding event, since documental description is only qualitative. An expanded view of the reconstruction for the 16^th century (bottom panel) reveals an outstanding agreement between the exceptional events documented during this period (black triangles) and the reconstructed phases of maximum discharge.

Figure 4. Tephroanalysis of sediments from the Gallipoli Terrace: time-depth relationship.

The depth at which a volcanic peak is found in the sediment is plotted versus the historical date of the corresponding eruption, expressed in years counted backwards from 1979 AD, i.e. the date of the core top. The straight line resulting from a linear regression fit to the experimental data is also shown.

Figure 5. ²¹⁰Pb activity as a function of the sediment depth (data from Bonino et al., 1993²¹).

The sediment samples for the activity estimation have been taken at 10 different depths with a sampling thickness of 1 cm. Dashed and solid blue lines are the total and excess activity, respectively. The black line is the least-square fit of the excess activity. ¹³⁷Cs activity is reported in the left-bottom corner.

Figure 6. Linear regression between δ¹⁸O and Po River discharge decennial components over the period 1917–1979 (blue line) and 1917–1965 (red line).